IIRC bullet proof vests function is not to just deform the bullet, but also to spread the force of impact over wider area, which in turn means you'll feel like punched after hit by a bullet and might get a big bruise, and bullet would need even more energy to penetrate the vest and the person himself.
Kevlar vest can stop a knife too, by deflecting or breaking it, whereas soft tissue wouldn't have done either of the two.
But a straight thrust might have a different effect, however it will require more power to punch through the vest when the energy is spread over a bigger area, so sloppy thrusts can be even stopped.
I've read about cases where the knife has been deflected or broken against typical bulletproof vests that cops wear.
Broken ones have been usually the ones used to cut bread....
With some basic logic, what makes difference between a knife and a bullet against the kevlar vest, is that the other is quite a light projectile which does not have a constant energy and the other will have a thrusting energy all the time along the path.
Those common bulletproof vests are only for small calibers anyway.. so the knife will have much more energy if thrusted right.
