It depends on your PC's speed, version of MAME and the particular game. V sync tells the screen renderer to "wait" until the monitor is ready to draw the next frame. If your PC is too slow, the next frame refresh on the monitor might have already started, which then means the renderer is waiting an extra frame. In this case, you're better off tolerating tearing, rather than losing frame rate. Just turn on frame rate (F11 ?) and judge for yourself.
OH and don't forget that various other things like special graphics filters, triple buffering, etc, etc adds to the processing, so if you want V sync, but lose frame rate, maybe try disabling some of those other extras first.