Bulletproof vests are designed to stop BULLETS not knives. It might take a little bit more force to stab through one, but its not really that hard. Bulletproof vests are actually quite malleable, and are made up of multiple layers of fabric and kevlar -- they help minimize the impact of a bullet by distributing its impact over a larger area. And, a key component that is instrumental is the design of kevlar is that the object must be traveling at an extremely high velocity. A blade can go right through a bulletproof vest because it was not designed or meant to protect against that kind of a weapon. They actually make stab proof vests ... they work a completely different way.
Edit: "Bulletproof Vest" is also somewhat of a misnomer ... they cannot offer 100% protection against bullets. Regardless of whether or not the vest actually succeeds in stopping the bullet you are going to be injured. It would be kind of like being hit with a frozen paint ball -- it will leave bruises, and can even break bones. Furthermore, the police version of a bulletproof vest your average beat officer wears is not designed to offer relative protection from rifled ammunition, nor from point-blank handgun rounds (especially higher caliber rounds). Military combat body armor is much different from that Police Officers wear ... it is designed to protect against rifled ammunition.
So it seems that it doesn't do too well with knives at least, not like you'd expect. It might be better with PVC, but I don't really want to find out