The simple answer to your question is that I don't know if you are doing harm giving the vaccine. I also don't know if you are doing a great deal of good. The veterinary literature still awaits the definitive word on FIP vaccinations. The reason FIP is so tough is because much of its destructive nature is due to the body's immune system reaction to the virus. In FIP, antibodies produced to fight off the virus end up causing a whole lot of the ill effects we see as disease.
Vaccinations typically are more effective at stimulating the humoral side of the immune system.
So, until more is known about the results of vaccination with the FIP vaccine, it is difficult for me to get behind it, even on a theoretical basis. There have been expermental studies of it that add to the confusion. Additionally, there are experts in feline diseases who have plenty of second thoughts. Occasionally my dog vomits a viscous liquid which looks kind of frothy and is a clear to yellowish color. I have never noticed any digested food in the goo.