We vaccinate all our cats against cat flu and enteritis as this will help them and if they do contract either of the above it will be much milder. A certificate for these will be necessary if you are putting your cat in a cattery. We don't vaccinate against Leukaemia as some cats can have a bad reaction but new owners can have this vaccination as an extra from their vet.
At the moment there is no vaccine against FIV although I believe research is being done. FIV is however not a killer and I have homed many FIV positives in indoor homes who are still fine after 12 years. The main thing is to get prompt treatment if the cat is ill.