Enable Borderlands Vsync through Inspector or Ini. See if it works.
Nope, still no sound prompt. And still not able to force vsync via D3DOverrider. Yes, I'm able to get vsync to work in the control panel and ini, but still.
I'd check if triple buffering was working in the Event History Panel but I can't seem to find this damn thing in Rivatuner.
Edit: The plot thickens. I just tested D3DOverrider on Mass Effect 1 and not only do I get the sound prompt on startup, I can also force vsync via D3DOverrider no problem. So it works as it should. What the heck is going on?
Bioshock - Sound prompt for triple buffering, but can't force vsync via D3DOverrider
Borderlands - No sound prompt, and can't force vsync via D3DOverrider
Mass Effect - Sound prompt, CAN force vsync via D3DOverrider, perfect
This is all using the latest Nvidia drivers (301.10 with custom inf) on a 560TI.