VSync is (definitely in OpenGL, I would assume in DX and DD) completely up to your video card driver. I know that with OpenGL, you can only set the attribute and hope for the best. In addition, you can ovberride the setting of vsync from applications in the driver's configuration utility.Does SphereGL have these issues in fullscreen? Sphere GL in fullscreen, for reasons unknown to me, is the only time it will run in doublebuffered mode.
It probably also matters how exactly an application asks for vsync.