You only have to set the game resolution, vorpX figures out the best headset resolution based on that.
How high the resolution should be depends. No one size fits all anwser here unfortunately.
You have to find a balance between performance and fidelity. What that means in particular depends on the game, your PC and last but not least your personal preferences.
As a rule of thumb: Go as high as you think it’s OK performance wise. For ‘Full VR’ mode you’d typically want a 4:3 res or something slightly narrower is possible. For immersive/cinema mode you want a 16:10 or 16:9 res.