OK weird one. Any game using D3D 8.1 will not allow vsynch on. I have it turned on ingame and globally. Particularly affected are Unreal Engine 2/2.5 games (UT2004, Unreal 2, Postal 2 etc). DX 9,10,11 - vsynch works fine. It's not just that engine, I tested with a game that allowed me to switch between DX 8.1 and DX 9 and vsynch works with 9 but not with 8.1. Using Windows 10 with an R7 260x.