Vibrato is an expressive device no flutist should be without, but one which is personal and variable. I think one of the best ways to understand your vibrato is to tape record your practice and listen carefully to the speed as well as the width of the vibration. Vibrato needs to have a musical context and correct execution: make sure that the throat is relaxed (the vocal folds need to be separated). Vary the vibrato according to musical expression but not necessarily within the same phrase. It may help to work on vibrato with a metronome to assure that it is even in the beginning stages of development. Listen to other great musicians to hear correct vibrato and various widths and speeds.