Can’t remember currently whether that’s the case for Grounded, but if vorpX can do it, let it do its thing and just select a preferred quality on the DirectVR page of the vorpX menu. VorpX will then figure out a resolution that works best depending on various settings, e.g. whether you play in FullVR or immersive screen/cinema mode.
Whenever this is possible for a game, there is an according message in the top left corner of the game window on start. To get the most out of this feature, check the Custom Resolutions section in the help.
If there is no such message, the general rule of thumb is 4:3 for FullVR, 16:9 or 16:10 for immersive screen/cinema. Ideally 1440p or better, depending on what your PC can handle without sacrificing too much performance.