Should be pretty easy to hack over to cairn or any other OSR / nsr system. The creatures for vaarn are pretty unique but nothing impossible and statting cairn creatures is easy anyway. The tension clock is built on top of the adventure (despair) with an underlying water and rations clock, so you’ll want to be tracking that to keep the pcs moving and searching. The vaults of vaarn srd is Creative Commons and free online (and linked throughout the pdf) so you can see what it’s doing under the hood if you have anywhere specific worries.