I read somewhere (don't remember where or if it's accurate so don't take my word for it) that by turning on vertical sync you're telling wow and your vid card to report the actual framerate produced, with it off, wow is showing the fps that it's sending to the vid card. Did the game seem to be running smoother overall with it off? I'll try it for a while, I'm actually waiting on the Azgalor pull right now, so we'll see if it works at least with one wow open and in a 25 man raid.

Update: With my graphics set to lower, not lowest, but lower, my fps did shoot to 50 before I stepped into hyjal, but I turned my settings all the way up and was getting 10-15 fps mid trash clear with infernals running rampant and all that other stuff. I'll turn it down for other stuff but I would guess that fps would go back up.