Some people have vibrato confused with a wobble in the voice. If a singer doesn't have enough breath support, their voice will start to tremble or wobble when they sing a sustained note. That is NOT vibrato. Actually, as my voice coach says, vibrato is not necessarily even part of a good singing voice. What you want with your voice is good tone quality. When instrumentalists use vibrato, they can improve the tone quality of the sound coming from the instrument. To talk about this with the voice doesn't really apply, because the singer doesn't have to do anything to cause "vibrato", they just have to develop good tone quality (and alot of this is learning to have good breath support).

Some people are born with naturally better sounding tone in their voice than others, but the tone can be improved by learning good technique. The term vibrato is very misunderstood when it comes to singing - most people confuse it with a wobble in the voice, which doesn't sound good, and really is a result of poor breath support. A wobble distorts the pitch, and this shows up especially in recordings. Don't try to develop a vibration in your voice. Get a good voice teacher and learn good breath support. You mention varying volume levels - steady breath support gives you the ability to vary the volume of your voice. An elegant ending of a phrase shows you that a singer has great support, instead of just dying at the end of a note because there isn't enough breath, or ending up in a wobble (because they are running out of air). Vibrato isn't the issue. With this input, go back and analyze the "line" that good singers have. One bad distortion of a singing line is a wobble. Develop a good line with your voice. A line = consistent singing quality throughout the song. Listen to singers that can sustain the same quality through a song and sound like themselves (not an imitation of someone else) throughout the whole song. Listen to the expression, the tone quality, the modulation of the voice, and find YOUR voice, don't imitate.

Alice