For now any screens using the Amiga's on board chips will use the original video outputs. Only RTG screens are displayed via HDMI.

This will change in an upcoming release.

If you have a scan doubler from Individual Computers and your monitor supports multiple inputs, the hardware banging resolutions go to that over VGA and Vampire goes over HDMI. If you don’t see it on HDMI you have to switch to VGA.

The scan doubler is shitty though: the quality varies with the monitor and it overheats. Jens knows about this because the manual mentions adding a small fan and since he was obviously aware of it, it should have been designed in.