Hi Corinne,
We don't often find ourselves vaccinating animals during their spay and neuter appointments, so I don't have anything to offer in the way of experience with the problem
In theory, the biggest concern might be the brief period of immune suppression that follows vaccination, allowing a post-operative infection to take hold. Post-op antibiotics and rigorous sterile technique may be minimizing that risk for you.
Otherwise, my main concern with vaccinations is when we give them to animals that either have obvious immune-mediated or inflammatory problems; or are on immune-suppressive medications.
If that is not happening either, then while I agree that vaccination at the time of spaying or neutering is an extra stress on the system that is best avoided. there may be no harm in it. I think that's the central question - you're looking for what's optimal, while your colleagues are looking for what can be tolerated. The answers to those two questions tend to converge as we get much older, but can be quite far apart for the young of many species.
Hope that helps,
Steve