If your flock has mareks, then there's not really a point in vaccinating broody hatched chicks. They've already been exposed during hatching and they need 2 weeks of quarantine after vaccination from any mareks exposure
They certainly ly can go blind. I have a hen that's in the double digits that gets wobbly on bad days and on her worst days did need to be righted once or twice. But on her good days, she is just like most birds