Another question then....if you mix vaccinated stock with unvaccinated, what happens.
depends on which vaccine is used and I am not sure which (if any) are live these days and its the live ones that carry a small risk of transmission.
The main 'problems' with vaccinated stock as I see it are:
If you do end up with something you've vaccinated against on your property you might be less likely to see it early with vaccinated stock. Remember vaccination doesnt prevent infection, just (hopefully) prevents full blown disease. So vaccinated stock can still pick it up and pass it on to non-vaccinated but not show any signs themselves and the vaccine isnt to 'blame'. And then there are those animals with which vaccination will be less successful anyway-its biology, nothing works 100% a 100% of the time.
The other problem is imho many of these commercial birds are pushed very hard from the outset and (apart from the odd one) seem to die quite young or not lay much after their second year. My last hybrid died at the weekend at 4.5 years old. The rest of that batch died or were culled in their second year.
The only way to keep them disease free is to keep wild birds away from them-and pheasants.