Vsync ALWAYS on. That is unless you LIKE rubber bullets. If you don't mind your bullets passin' thru the nme, then leave it off.
Variance spikes, that look like a heart monitor, consistant up,down,up,down are a symptom of someting running in the background (per. skuzzy) and I would do the usual spyware checks, and kill everything in the your TSR's that is not needed.
Can't think of a single reason why it would be connected to your vsync. Anyone?