Vaccination has greatly reduced the incidence of life-threatening infectious diseases within the cat population. Whether or not your cat needs to be vaccinated will depend on its lifestyle and risk of infection, so make sure you talk to your vet.
Cats with FIV have a weakened or suppressed immune system and may be at greater risk of developing other infections if they are exposed to them. FIV positive cats can be vaccinated to offer some protection. Speak to your vet for more information.