The important setting for vorpX is the resolution you run a game with, not the SteamVR supersampling, which only comes into play in a second step when the already rendered game image is sent to the headset. This is quite different from how native apps work.
If you want to raise the image quality, don’t mess with SteamVR, raise the resolution of the game. For many games vorpX can handle that with its DirectVR feature, otherwise you typically want a 4:3 res for FullVR (unless you have a Pimax 5K/8K) and a widescreen res for cinema/immersive screen mode.
I’d also highly recommend to check the ‘Custom Resolutions’ section in the vorpX help that explains how to add resolutions to your PC that it does not support yet.