Vocal vibrato is like garlic... some songs/dishes call for more, some less. Sometimes you want artichoke garlic, sometimes elephant garlic - sometimes you want faster wave vibrato, sometimes slower. What you want in both the singer and the chef is... control!
Garlic aside, in my practical experience, these are the two most effective remedies for control of vibrato:
1. The diaphragm.
You must enable the diaphragm to control itself so the air sent to your vocal cords is controlled (enabling breath control). The diaphragm works in conjunction with the automatic nervous system. What you can do for it consciously is to widen the base of your ribcage, giving it a good taught stretch. This helps it control the air bounce it sends upwards.
2. The mental intention.
Again.. the diaphragm is directed by the automatic nervous system. What you can consciously do is to fully intend a certain amount and speed of vibrato. Just 'think' it clearly... do you want to hold a tone straight? Want to start straight and end with a 'shimmer' of gentle vibrato? Start with vibrato, go straight and/or shimmer out at the end? Want a big wavy vibrato? Be proactive... INTEND what you want to do. This gives your automatic nervous system its marching orders.
Special note... you can sometimes reinforce your mental intention by looking at your hand moving up and down, at the speed you want your vibrato.
And btw, garlic is good for your voice!