I think the kitten is a Birman, look at his feet. A Persian has a flatter face and more of a squashed nose. Birmans are cute though. why dont you post a picture of your cat.
They are fairly common. Do a search for Vaccination Associated Sarcoma in Cats or VAS. Also this website has a heap of information www.catshots.com I found out about VAS while researching adverse vaccination reactions as my cat had an bad reaction. It is well worth knowing about these side effects.