Late: I'm so sorry. Believe me, amputation will much harder on you than on your kitty... they really do amazingly well with stuff like this after the first initial "get used to it" phase. They really are amazing creatures.

As for future vaccinations: if they are indoor kitties, you really can limit exposure to vaccinations by not having FELV vaccine (is passed cat-to-cat in close range -- just be aware to test any incoming kitties before exposure to your current ones); adult cats only really need FVRCP every 3 yrs. Rabies is up to the state, but most are every 3 yrs after initial kitten and one yr boosters.

We have been vaccinating in the limbs for just this reason -- if something horrible happens, and the only alternative is amputation, at least is has only exposed a limb, and not the middle of the back where you cannot get good margins to get rid of it.

It sounds like you have found some wonderful vets to consult with.
I wish your boy all the luck in the world, and great healing vibes for surgery. He will be tougher than you think.

Ask for (no... demand) excellent pain meds -- fentanyl patch placed the day before will work wonderfully for him (takes 6 to 12 hours to kick in). Buprenorphine can be added to this mix in addiition for added pain control. It is so important b/c kitties hide their pain.

Good luck to you and him with everything.