I usually keep vsync on in games, cause tearing irritates the hell out of me. But if your card can't keep up and only does, say, 50 fps while your monitor refreshes at 60 Hz, then your FPS could drop down to 30 with vsync on. That's the only time you'll take a noticeable performance hit, and obviously if your card can't keep up with your monitor there's no reason to have vsync on.