Vibrato is a symptom of a free functioning vocal instrument. It is not something we make or produce, but shows up when all is well; not too fast and not too slow!
Vibrato is natural. When it shows up, then we can allow it to simply be. As we allow it to be, then our voice is both working well and relaxed. We can sometimes vary our vibrato with a more straight tone – all for the purpose of expressive effect.
Our vibrato can even vary its speed, a little more or a little less, to indicate excitement or sadness. Remember, all of this is nothing we do, but something that happens as vocal muscles respond to our heart song.
So, sing your heart out, and let the vibrato be free and natural.