When you are using the DK2 as main monitor, did you set vorpX to “Use system settings”? That is vital. If it’s not done, vorpX will still try to force the game to the Rift.
This should definitely always work, in this case DirectX just does what it always does, meaning opening games on the primary monitor whatever that is.
Apart from that using the Rift as primary monitor normally should only be necessary in some special cases.
Every few months I get a report of mysteriously swapped display names in the config app, maybe that’s your issue. Trying “Show on [your other monitor]” instead “Show on Rift DK2” may be worth a try. Never had the time to look into this so far unfortunately.