Sure, that part is well understood. But triple buffering doesn't avoid the need for vsync. If the VBL interrupt isn't reliably available on all the VGA cards

Sure, that part is well understood. But triple buffering doesn't avoid the need for vsync. If the VBL interrupt isn't reliably available on all the VGA cards out there, you are still going to have flip pages from the foreground at some point, and you would rather not do that outside the blanking interval. I suppose you could poll the CRTC every so often during the game loop or rendering process, though. That must have been how it worked.