Any vets on here?
Don't know, but I found a tame one and asked her. She basically said that this shouldn't be a problem since most vaccines are 'dead', i.e. they can't transmit the actual disease, so the vaccinated hybrid hens would not be carriers of that disease (the vaccine didn't give it to them, and also made them immune from catching it from another source).
Note, she didn't say it couldn't happen (depending on the specific disease / vaccine), but did say that in her opinion it was pretty unlikely, and there are probably more important things to worry about in life

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So Moleskins, Very sorry for hijaking your thread! I suggest that you buy whatever birds you like the look of, but keep in mind factors like hardiness and laying rate. Actually, when we started out, we went to a breeder, sat down in his pen and stuck our hands out full of grain. Funnily enough, the birds that chose us turned out to be real crackers!