Vibrato is natural. When it shows up, we can allow it to simply be. As we allow it to be, then our voice is both working well and relaxed. Sometimes we can vary our vibrato with a straighter 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 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.