Read more about Lower Vital from the works of the Mother and Sri Aurobindo.
What is the Lower Vital?
From the navel downwards is the reign of the vital passions and sensations and all the small life-impulses that constitute the bulk of the ordinary human life and character. This is what we call the lower vital nature. 
It is usually ignorant and obscure, and is the seat of small desires, greeds, passions and enjoyments. 
Lower Vital Centre
The centre between the navel and the Muladhara commands the lower vital (physical desires, small greeds, passions etc.). 
In the abdomen is the lower vital centre. It is in these two [vital centre at navel and lower vital centre] that there is the origination of desire—but desire rises and becomes emotional in the heart and mental in the higher centres above. 
In the process of our Yoga the centres have each a fixed psychological use and general function which base all their special powers and functionings… the abdominal centre—svādhiṣṭhāna— governs the lower vital; 
The lower vital is concerned with the pettier movements of action and desire and stretches down into the vital physical where it supports the life of the more external activities and all physical sensations, hungers, cravings, satisfactions. The term lower must not be considered in a pejorative sense; it refers only to the position in the hierarchy of the planes. For although this part of the nature in earthly beings tends to be very obscure and is full of perversions,—lust, greed of all kinds, vanity, small ambitions, petty anger, envy, jealousy are its ordinary guests,—still there is another side to it which makes it an indispensable mediator between the inner being and the outer life. 
The lower vital is occupied with small desires and feelings, such as make the greater part of daily life, e.g. food desire, sexual desire, small likings, dislikings, vanity, quarrels, love of praise, anger and blame, little wishes of all kinds—and a numberless host of other things arise from it. 
Nature of the Lower Vital
Ignorance and Arrogance
The lower vital nature with all its pretentious arrogance, ignorance, obscurity, dullness or incompetent turbulence, standing for its own prolongation, standing against the descent, refusing to believe in any real reality or real possibility of a supramental or suprahuman consciousness and creation or, still more absurd, demanding, if it exists at all, that it should conform to its own little standards, seizing greedily upon everything that seems to disprove it, denying the presence of the Divine—for it knows that without that presence the work is impossible,—affirming loudly its own thoughts, judgments, desires, instincts, and, if these are contradicted, avenging itself by casting abroad doubt, denial, disparaging criticism, revolt and disorder. 
Attachment to Old Self
The difficulty in the lower vital being is that it is still wedded to its old self and in revolt against the Light;...[it has] no such will and hardly any idea even of what true surrender is. When the lower vital assumes this attitude, it takes its stand upon a constant affirmation of the old personality and the past forms of the lower nature. Every time they are discouraged, it supports and brings them back and asserts its right to freedom—the freedom to affirm and follow its own crude and egoistic ideas, desires, fancies, impulses or convenience whenever it chooses. 
Dissimulation and Falsity of Speech
...it is not mere exaggeration or a false use of the imagination embroidering on the actual truth that is marked in the sadhaka, but also a positive denial and distortion as well as a falsifying concealment of facts. This he does sometimes to cover up his disobedience or wrong or doubtful course of action, sometimes to keep up his position, at others to get his own way or indulge his preferred habits and desires. 
The lower vital is very slow in listening to reason—at least when reason is on the side of Truth; although it is very pleased to listen to the mind when the mind justifies the lower vital's wrong movements. In fact the lower vital moves on its own lines, not according to Truth or reason. It is only the insistence of a stronger Light that can enlighten it by changing it in its own substance of consciousness. 
Disobedience and Indiscipline
This lower part of the being is always random, wayward, self-assertive and unwilling to accept the imposition on it of any order and discipline other than its own idea or impulse...This habit of disobedience and disregard of discipline is so strong that it does not always need to be deliberate; the response to it seems to be immediate, irresistible and instinctive. 
A dangerous habit of constant self-justification. His [sadhaka’s] mind rushes at once to maintain his own idea, his own position or his own course of action. This he is ready to do by any kind of argument, sometimes the most clumsy and foolish or inconsistent with what he has been protesting the moment before, by any kind of misstatement or any kind of device. 
Elevated Form of the Lower Vital
There is behind all the vital nature in man his true vital being concealed and immobile which is quite different from the surface vital nature. The surface vital is narrow, ignorant, limited, full of obscure desires, passions, cravings, revolts, pleasures and pains, transient joys and griefs, exultations and depressions. The true vital being on the contrary is wide, vast, calm, strong, without limitations, firm and immovable, capable of all power, all knowledge, all Ananda. It is moreover without ego, for it knows itself to be a projection and instrument of the Divine; it is the divine Warrior, pure and perfect; in it is an instrumental Force for all divine realisations. 
Why Transform the Lower Vital?
For Stability and Decisiveness
It [failure in whatever one tries to do] usually comes from a certain instability in the lower vital which does not give a consistent support to the Will, but is restless and fluctuates from one interest to another. It does not mean an incapacity for success—usually one who has that could succeed in many directions, but the fluctuation prevents sustained success in any. It is a defect that has to be got over and can be got over. 
For Peace and Ananda
Love should be a flowering of joy and union and confidence and self-giving and Ananda,—but this lower vital way is only a source of suffering, trouble, disappointment, disillusion and disunion. Even a slight element of it shakes the foundations of peace and replaces the movement towards Ananda by a fall towards sorrow, discontent and Nirananda. 
Whatever there may be in these higher parts, aspiration to the Truth, devotion or will to conquer the obstacles and the hostile forces, it cannot become integral, it cannot remain unmixed or unspoilt or continue to be effective so long as the lower vital and the external personality have not accepted the Light and consented to change. 
... the consent of the lower vital must not be only a mental profession or a passing emotional adhesion; it must translate itself into an abiding attitude and a persistent and consistent action. 
How to Transform the Lower Vital?
There is only one way of escape from this siege of the lower vital nature. It is the entire rejection of all egoistic vital demand, claim and desire and the replacement of the dissatisfied vital urge by the purity of psychic aspiration. Not the satisfaction of these vital clamours nor, either, an ascetic retirement is the true solution, but the surrender of the vital being to the Divine and a single-minded consecration to the supreme Truth into which desire and demand cannot enter. For the nature of the supreme Truth is Light and Ananda, and where desire and demand are there can be no Ananda. 
There is certainly a sincere will to change. This resistance of the lower vital is usual in everybody—it is the main difficulty in Yoga. If it were not there, the change would be easy. But once the steady mental will is there, it is a question of time and steady sadhana. With that the change is sure. 
It is always their [the lower forces] endeavour to rise up and get the sanction of the mind and higher vital—or if they cannot do that, to cover them up so that the lower nature may act in its ordinary way without being pressed by the higher consciousness to change. The first thing necessary for the sadhak is not to give any consent of his mind or higher vital and to keep them from being covered. If that can be done, then it becomes possible to push the lower forces out of the lower vital and body and not allow them to return. It is the Mother's Force which you feel—for all the higher forces are hers. 
... the more you strive to realise, you will discover in the nature—the lower nature, that is, the lower mind, the lower vital, the physical—how much pretension, sham and ambition there is…. One can use any…. The desire to put on airs: all that must be eliminated, absolutely, radically, and replaced by a sincere flame of aspiration, of aspiration for the purity which makes us live only for what the Supreme Consciousness demands of us, which makes us able to do only what it wants, which makes us do only what it wants, when it wants. 
By bringing down strength and calm into the lower vital (region below the navel) [fear can be eliminated]. Also by will and imposing calm on the system when the fear arises. 
An occasional sinking of the consciousness happens to everybody. The causes are various, some touch from outside, something not yet changed or not sufficiently changed in the vital, especially the lower vital, some inertia or obscurity rising up from the physical parts of nature. When it comes, remain quiet, open yourself to the Mother and call back the true condition, and aspire for a clear and undisturbed discrimination showing you from within yourself the cause or the thing that needs to be set right. 
Role of the Psychic Being in Transforming the Lower Vital
The psychic when sufficiently developed can be strong enough to make the preliminary clearance [of the lower vital].
It is the supramental alone that can transform the material being, but the physical mind and physical vital can be very much changed by the action of the psychic and of the overmind. The entire change however is made only when there is the supramental influence. But for the present the psychic is the force that may be relied on for the preliminary purification of the lower nature. 
Read more about Lower Vital from the works of the Mother and Sri Aurobindo.