Vaccinating a FIV+ Cat

So my cats have their annual vet visit tomorrow and they're both due for their vaccines. Rigby (FIV+ if you couldn't guess from the title) hasn't had any shots since 2012, before I adopted him (he's been to the vet just no vaccines). He's 99% indoors only. He likes to door dash and has gotten out for about an hour maybe twice a year. I'll also take him for walks around the apartment property.

What should I know about vaccinations and FIV? I'm worried about stressing his immune system, but some things are saying that it boosts immune systems which would be good, right?

Any input or nudges in the right direction of solid info would be greatly appreciated! TIA!

Could you ask your vet to do a titer (?sp)--to see if he has the requisite level of antibodies?
I recently TNRd Gandolf, a very feral, old (10 years) cat, who tested positive for FIV+. The spay/neuter clinic didn't hesitate to vaccinate him against rabies. But "Gandolf" is a strictly outdoor cat, and there are racoons in the area. So in Gandolfs case, the pros outweighed any cons.