It is quite simply impossible to learn how to sing from a book or a website. The written word can be motivating and clarifying (or perhaps confusing....) but we need help from another pair of ears. Unlike other musicians, singers hear a different sound on the inside to what comes out. There are certain aspects of singing which are more straightforward to describe than others. But the above comes with so many variations of interpretation and nomenclature, that I tread very carefully and only touch the surface.
Vibrato is a series of pulses along the vocal folds caused by the actions of antagonistic muscle groups in the larynx as they hold the vocal folds in the requisite state of tension for each pitch and intensity required. The range of these pulses is between 5 and 8 times per second in what is generally accepted as natural and aesthetically pleasing in classical singing. Anything slower is perceived as a wobble and anything faster as a tremolo. As vibrato is formed by a perfectly natural physiological action, it is integral to correct and healthy singing. Any suppression of vibrato violates physiological law and although it is cultivated in some quarters, it can only be achieved in breach of that law by holding the anchoring muscles of the larynx in a state of rigidity. It will cause undoubted tension - resulting in eventual, if not immediate, vocal problems.
The true vibrato is subtle, regular and pleasing. If it becomes too prominent, it disturbs rather than enhances the tone and if it becomes irregular, it irritates rather than lulls it. It does, however, start to increase in prominence with dynamic swelling (but always in perfect relation to the quantity of tone) and it speeds up as the pitch gets higher. Consequently, it must be recognised as an enhancer and an excitor in singing. Its true nature can only be achieved by what is fundamental to all aspects of good tone production - the perfect balance of breath pressure to vocal fold tonicity. Indeed, the vibrato is a useful monitor to the health of the vocal organ. If it becomes supressed, too intrusive, too fast, too slow, too wide in its fluctuations either side of the pitch and/or irregular, you can be sure that there is some basic fault in tone production.
If you want to hear a perfect vibrato in action, you could do a lot worse than listen to Pavarotti. What we hear is a rich, golden, warm, sunny, generous sound that thrills and beguiles at the same time. We probably don't even notice the vibrato. And that's how it should be. It is there to enhance the tone - not to dominate it. If we listen out for it, we're aware of a pleasant throb within the resonance. It increases in perfect proportion to the expansion of tone and it fades away with its diminution. As the voice ascends in pitch and gets more exciting, its rate speeds up to make the timbre even more so. Listening to singers such as Pavarotti is an ideal lesson in how vibrato functions. Here Pavarotti exemplifies the above to perfection.