The thing with MAME is that you dont really want vsync on at all. A lot of arcade games run at some weird framerates, you will often see framerates like 55, 58, even 61, 62 etc. The point is there was no standard and as they were on CRT's it didnt really matter what the exact framerate was as they still worked fine on a CRT. The issue now is we are all using flatscreens which have specific framerates like 24, 30, 60 etc and anything that deviates from those will tear. The problem is though if you turn vsync on those games will lock to a 60 refresh and if say a game originally ran at 55 it will be sped up to 60 to match the refresh of the screen, so some games will run faster than they should which obviously isnt correct. I personally leave vsync off and let the games tear if they need to in order to keep the original game speed correct.
Hope that helps somewhat.