Hello,
Just in case: Did you follow the DK2 Setup Guide in the vorpX help? It explains how to set things up.
In GTA V you can alternatively select the Rift directly, which might be the easier way if you are new to vorpX. GTA V allows to select the output monitor in the game options.
You should still read some of the guides in the documentation, they explain quite a few things that make your life easier.
While I’m at it: here is a settings guide for GTA V, that might be of interest once you got it basically up and running.