A few games already should work in Direct HMD mode, Skyrim for example. This may be system dependent though, so consider this *very* experimental.
There is a another way though:
1. Set the Rfift to Extend Desktop
2. Set the Rift as secondary display in Windows
3. Set vorpX to “Show only on Rift DK2”
Just be aware that this does not work with everything. For some games you will have to revert to the DK2 as primary display.
The above setup is explained in greater detail in the DK2 setup guide in the vorpX documentation.