before you get to developing a good vibrato on the guitar remember that it is derived from the human vocal vibrato and listen to great vocalists from all genres to understand how vibrato is used to sustain a note. And if you understand vocal vibrato and are musically sound enough to understand what is IN and what is OUT...All you have to do is emulate that.And practice hard.Remember to apply the vibrato only to the end of the phrase and relevant places,overdoing it is not natural.