Hi I noticed no one has been able to help you out yet regarding the cats under your house. Bravey gave you a great way to get the smell out of the vent.
Now this is somthing you may or may not want to try, but I did it to get rid of some rats under my house but it took a couple of goes.
Because I have a swimming pool I always have a couple of containers of Dry Chlorine for the pool. What I did was heaverly sprinkle chlorine all under my house. The rats hated and moved out. Plus it helped reduse that awfull smell.
The warnings are.. wear eye and breathing protection. The fumes are strong at first but it dosn’t take long to disappear.
The cats are going to hate it and won’t be able to live there. The good thing is the chlorine is going to kill any bacteria that has formed under the house and help get rid of the smell. Let me know how you go.
Regards Noelene