So when you bend up the string dies out? Sounds like the strings are fretting out due to the fretboard's radius. Meaning, raise your action a pinch and your problem will be solved.
If not, then please describe in more detail where/how you bend vibrato. Fretting out mainly happens on the higher strings past the 12th fret, so if you are doing stuff elsewhere and still get deadened note it may be some other factor. Either way, raise your action a bit, as this solves most fretboard problems anyways.