Sure, as long as they don't stab anywhere but the chest.
Live Trade Show Demonstration Of Stab-Proof Vest

This is a video of PPSS Group’s CEO donning one of the company’s stab-proof vests for a live demonstration at a trade show. The vest is made of a carbon fiber composite, and differs from bulletproof vests which are designed to flatten bullets but can allow a knife blade through. In the video, he’s repeatedly attacked with a knife, ice pick, machete, and aluminum baseball bat. He does side-step that last bat swing though. I’m gonna have to call that a strike. While certainly an impressive vest, I can’t help but think of Wu-Tang Clan’s 1992 debut single. “Protect Ya Neck.” Face too, that man has a knife.