I'm playing a challenge called the vampire coven challenge, and I'd really like to start a coven in the future. I want this coven to be part of my active family, but when I leave Oasis Springs I want them to stay behind and live there. In the base game this isn't possible, is there a mod anyone knows about that will make this possible?

I do not play ITF but the NRaas Traveler Mod can do what you are asking. Perhaps someone in here can advise if it works well with ITF.

The only thing "Treat as Vacation" does is toggle the ability for sims to go to work/school in a world that is not their currently assigned homeworld. Not sure what this has to do with horses. Unless you routinely send your horses to work/school?

I'm not an ITF player either, but I have to agree that Oasis is not designed to become a homeworld for currently active sims, at least not any you ever expect to see again.

Huh. I'm going to check this the next time I am in game. My sims happen to be on vacation in Monte Vista and, although I haven't tried to place any pets in the inactives' homes myself, can't help but notice that SP is rapidly allowing them to adopt new puppies and kittens like crazy. Are you running SP on these non-ITF vacation worlds? And how are you trying to add the pets, with MC Adopt Pet, Add Sim, some other way?

Thanks, I'll try the traveller mod. I knew it was a long shot that there would be, so thanks for trying to help