That works somewhat different with vorpX. The SteamVR resolution only affects vorpX’s second rendering step and is determined automatically by the game resolution set in a game’s options menu. So that’s what you want to change for a crisper image.
In some games vorpX can handle resolutions automatically by selecting a quality preset in the vorpX menu. Whenever that’s the case you see an according hint in the top/left corner of the game window as well as in the headset.
If that is not available, you can set any resolution you like manually in the game options. For FullyVR mode most of the time 4:3 resolutions are the best choice to avoid rendering pixels that never show up in the headset, for cinema/immersive screen mode widescreen resolutions are usually best.
You may want to add additional resolutions to your PC that it can not display normally:
I’ve found for a few games, like Ralf said, if you want a resolution higher than your desktop, you have to not only add them to the list of resolutions in your video control panel, but then set the desktop to that resolution. Example: Sonic Racing Transformed. Have to increase the desktop resolution to run the game at a higher resolution. Others it doesn’t seem to be necessary