There appears to be a genetic variation that makes some cats susceptible to vaccine induced fibrosarcoma. However, some cats get fibrosarcomas in places they were never vaccinated; a friends' cat developed one on the bridge of its nose. The American Feline Practioners recommend that strictly indoor cats only be vaccinated every 3 years. The also specify the location of vaccine administration so that more work can be done on just which vaccines are implicated. The usual proticall is for the rabies vaccine to be administered in the right hind leg, the leukemia vaccine (which should only be given to outdoor cats) in the left hind, and the feline panleukopenia (distemper in lay terms) rhinotracheitis, Calici,& Chlamydia combos in the scruff. In most states, the rabies vaccine is required for public health reasons. There is a school of thought that the adjuvants (local irritants that increase the blood flow to the injection site and improve the immune systems exposure to the antigen (the active part of the vaccine)) may be the cause, so you may want to ask your practioner about adjuvant free vaccines.

There is a product called Acemannen that supposedly shrinks the tumors. The results at the clinic I worked at were not exciting or encouraging. My boss usually recommended limb amputation as the tumor is locally very invasive. I would investigate some of the veterinary oncology centers or at least the nearest Veterinary School. Most Veterinary Schools offer far more extensive treatment options than your private veterinarian can. Ask your veterinarian for a referral. (No, they won't take it as an insult.)