Good for you!!! That is a great site with a lot of very good information. I use it for research myself.
A slight correction though... even if you have multiple cats, if they are ALL strictly indoor kitties, then none of them should need the FeLV vax. None of my 6 cats gets it.
Now to answer your prior question, yes there IS potential for harm to your kitty from vaccines. Vaccine related sarcoma is a form of cancer related to vaccinating (in cats only).
I have actually seen articles quoting a much lower rate of protection from the FeLV vaccine than 75%. I cannot remember where I saw it, so I won't quote the number. It is really important to be informed about the things that affect your pet's health, from food to medical care, vaccinations, indoor vs. outdoor, and even the litter you use.
God bless 'em, vets don't always know it all.