Yeah, but spider silk is more elastic than Kevlar. Toughness is defined as the total energy you need to break a strand. Spider sil is tough because it elongate a lot before breaking, as such you need a lot of energy to break it. So yeah, your spider silk bulletproof vest won’t let any bulet go through, but it’ll bend around them until bullet and vest are both stuck somewhere in your precious, delicates, internal organs.