Practice with a metronome; set it to 60 bpm to start, and do one vibrato "cycle" every two beats (bend it up and back down). Work your speed up gradually to one cycle each beat, and then start shortening the beats (80 bpm, 100 bpm). When you're comfortable at 100, drop the metronome back to 60 and do eigths, working your way back up; then do the same for 16ths.
After you get good (16th's at 120 bpm, say) try adding some vibrato on top of your bends...
"There's such a fine line between genius and stupidity."
--David St. Hubbins