Problems with basic Bends and Vibrato

I have a small problem, that I hope someone can help me with... Whenever I try to bend a string or use vibrato during a riff or lick, I always end up muting the string I'm bending, thus not getting that special sound (or any sound at all). Is there some tutorial on bending and vibrato that I may have missed and if so, can someone please guide me to it?

Thanks in advance :)