Some bullet proof vests are made to be both bullet proof and stab proof as well.
But some other bullet proof vests are only partially stab proof and are not guaranteed to stop a knife but usually they will.
If you want true and guaranteed protection from a stabbing then you should buy a bulletproof vest that is also made to prevent stabbings.
The bullet proof vest will say that it's both bullet proof as well as stab proof and will protect you from stabbings.
However bullet proof vests cannot stop all bullets or shotguns from penetrating through them.
Some shotguns can go right through a bulletproof vest however it does still offer some protection against the shotgun.
Still even with a bulletproof vest you can still be shot in the head which can also be very deadly.