For a cat that spends any unsupervised time outdoors that isn't in a completely isolated enclosure, I would recommend the same course of vaccinations as an outdoor cat - it only takes one incident to transfer a serious disease, and if you aren't standing right beside your cat at all times outside you can't control what the cat comes in contact with.
Speaking in general, the way I decide for myself whether or not to give a vaccination is whether the risk of the shot itself (because that exists) is greater or less than the risk of them being unvaccinated against a disease. For my indoor cats, I do rabies and distemper vaccinations only.
Rabies vaccination is required by law here, and in my state it's pretty strictly enforced. My vet isn't allowed to let an animal leave the office without them either vaccinating or the owner presenting proof of vaccination, and if my cats were ever found to be out of compliance with the law and they had a run-in with an animal or person they would be put down. It's just not worth the risk for me - even though it's unlikely my cats would be exposed to rabies, the legal issue is still a concern.
I do distemper vaccinations too because distemper is so contagious. It can be spread via a human's clothes or shoes, so my cats wouldn't actually have to have direct contact with another animal to get sick or pass the disease on. This is especially an issue for my particular cats because I come into contact with pets of unknown health status at the animal shelter where I volunteer, but I would do this anyway because I feel they are at risk of contracting distemper even as indoor cats.
I do not, however, do feline leukemia vaccinations, because the odds of my cats ever contracting it would be negligible given how it's spread (and the law doesn't require it,) so for that one the risk of the vaccination would be greater than the risk of the vaccine.
Basically, there's no one right answer, and you have to weigh the risks on both sides and decide which is more problematic for you.