In the old VorpX there was an option to force the game onto a certain monitor, so that you didn’t need to have the Rift as primary monitor. This option appears to be gone in the new version?
In case you didn’t do that already, please check the DK2 setup guide in the vorpX documentation. It explains the full setup in three easy steps including images.
In that case please check the DK1 setup guide in the documentation. In short: The easiest and also most compatible way to use a DK1 is setting Windows to ‘duplicate displays’ and vorpX to ‘Use system settings’ display wise.