One of my cats has to have her vaccines "split" meaning she gets her rabies shot a number of weeks after all her other shots. She had a reaction a number of years ago, and now has a small lump between her shoulder blades. I do not think it's cancerous as it has not changed in size over these years. The vet always checks it when she goes in for her checkups. [ She has just got a clean bill of health after a geriatric blood panel, and urinalysis. She is 14].
We may have caught this in time from it developing into the vaccine induced sarcoma that TheAntiPam's cat Daisy has, or something similar. I certainly hope so.
But when I went in for the blood panel, my vet told me that there is now a "recombinant vaccine?" available that does not use the adjuvant that the previous vaccines used, and that she may be able to have her vaccines all at once again. I don't know any more about it at this time, but what I understood is that this new type of vaccine may minimize the risk of this type of reaction. I suspect it is the adjuvant that causes the cancer. (If anyone really KNOWS, please let us know?)
Hope I haven't rambled too much here....

All the best to your Daisy, TheAntiPam.