Many DOS games run the monitor in a mode with a 70 hz refresh rate. So by default we just emulate that which means it’s unlikely to be able to use vsync (because your actual monitor is unlikely to be set to a 70 hz refresh rate). But you can to go System > General > Force Output FPS and set it to match the refresh rate of what your actual monitor is set to.

If you go to Settings > Performance > Advanced > Show Performance Statistics and set it to “Detailed information” the emulator will show some stats on screen including the current resolution and refresh rate of the emulated screen. For example in the console it will say “[email protected]” meaning by default (unless Force Output FPS is set) it will draw to the screen 70.09 times per second.

Keep in mind that some DOS games might actually draw to the screen in a manner that will look like it’s a sync issue of the emulator. But in those cases it might actually be accurate to how it looked on original hardware. Speeding up or slowing down the emulated CPU might help but it could be tricky to dial it in perfectly. I think back in the day both developers and players cared a bit less about screen tearing, they were amazed enough if anything was drawn to the screen :-)