vorpX first renders the game, then afterwards to the headset, so you need to make sure games run at a high enough resolution.
If you insist on doing things manually for some reason, 1440p resolutions (e.g. 1920×1440) are a good choice and provide roughly 1:1 pixel mapping. You also have to take care of FOV yourself in that case.
However, if you allow vorpX to fully do its job and let it adjust the resolution and FOV, which it can do for a lot of games, you can simply choose a preferred quality on the Direct VR page of the vorpX ingame menu. Check the “Custom Resolutions” section in the help to take full advantage of this feature.