My inderstanding is that POL pullets are either vaccinated, or not, by the POL stage, and you just have to act according to what you perceive the risk to be. I don't think its an option for the vet to vaccinate older birds, as I believe they have to have their vaccinations at certain ages, when they are chicks, like children do. In ay case, vaccines come in large batches which cannot be stored once opened and are expensive for the full suite if only one or two doses are used, which is why many small private breeders don't use them routinely. When you buy a typical hybrid layer POL, she will have been reared in a large commercial setup where all the chicks are fully vaccinated as a large group at the appropriate ages. I know some private breeders do vaccinate, at least for Mareks, and I think Foxy would be able to advise about this.
Just remember that yes, there are always going to be potential problems in introducing any new birds to an established flock, and the possibility of vaccine crossover is only one of these. As.always your choice of good healthy, well-reared stock is of the first importance. At least, commercial hybrids have been reared carefully,they wouldn't find a large-scale market if they weren't fully vaccinated against a whole range of possible diseases. So, as a hobby keeper who just has a few birds mainly as pets and for eggs, they tend to be an excellent choice health wise, and if you just stick to fully vaccinated hybrids, there should be no problems. Similarly, if you can find a good private or smaller-scale breeder of really healthy stock, there is no reason why it shouldn't be perfectly possible to mix vaccinated and unvaccinated birds with no problems at all. My own small flock of mixed birds has so far stayed healthy through several generations of imports, and I have been advised by Nick at Chalk Hill Poultry that the dangers of vaccine crossover problems are very small compared with other potential problems from newly- imported birds.