All cats, even indoor only ones, need to be vaccinated! First, some cats may get outside and come into contact with wild animals or diseased cats.
Second, some viruses spread through the air or may be brought into your home on peoples clothing or shoes. Thus, indoor cats should be vaccinated for feline rhinotracheitis, calicivirus and panleukopenia. The schedule to be determined by your veterinarian.
Third, wild animals will sometimes enter the home. We had several instances in the past year of bats entering homes, lofts and apartments; putting owners and animals at risk. Several of the owners needed Rabies vaccinations, and their cats needed to be quarantined; because their rabies vaccinations had been allowed to lapse.
Therefore, for your safety, indoor cats should be kept current on their Rabies vaccination.