I personally think that sounds have samples with vibrato are problematic.

Vibrato is mostly done in sync with tempo. So, those samples will sound good, if you are playing close to tempo when they were sampled in. but if you shift keys, you are actually making the sample faster, so the vibrato does NOT sound natural.

Unless you sample each key for each tempo, you can't get a universal good sounding vibrato samples.

I agree with justforfun... the modulation wheel should be used to get the vibrato effect.