Watch the Blues Bends online guitar lesson by Jeff McErlain from Take 5: Blues Bends
Bends and vibrato are parts of a whole, as they're basically they same technique. I would highly recommend practicing them individually.
When practicing bends, as I describe in the video, fret the note you wish to bend, then fret the note you wish to bend to. Get both of those pitches in your ears, and do that a few times. Then, bend the first note until it sound the same as the 2nd note you fretted earlier. Go back and forth between these two and pay close attention to the pitch.
Also, really concentrate on proper left hand position like I have described in the video. To bend strings well, you ned some good left hand strength and control. The technique is extremely important!