Re: Tearing and Composite
Yes, the sync-to-vblank options in nvidia-settings dont' have any effect on VDPAU (with the exception that the GL setting affects any compositing window manager that's displaying VDPAU windows, but you aren't)
One more question: Since you're in TwinView clone mode, do you only see tearing on one display, or both? I assume you've only been looking at your external monitor up to now?
Let's try another test: Run glxgears or some other OpenGL application. Can you see any tearing on either monitor? What if you change the environment variable GL_SYNC_DISPLAY_DEVICE between DFP-0 and DFP-1 before running your app; does that affect anything?