I'm not being derogatory towards anyone, but some breeders will tell you birds are fully vaccinated when they aren't!
I did buy some water soluble vaccines 3 years ago. The day old chicks we got at the hatchery were vaccinated against Mareks anyway, and I vaccinated them over the course of 2 weeks against Infectious Bronchitis, Newcastles disease, avian encephalomilitis and another one. when When people came to buy the chickens, I gave them a sheet telling them all about the vaccinations, where they were bought, when they were bought, and info like the company purchased from and the number they give so the vaccine can be traced back.
Not many people do this. I don't do it anymore as it was a bit expensive, wasteful and no one seemed bothered whether they were done or not!
So you could try asking a few questions like what I've mentioned above and see what sort of reaction you get from the breeders!