Pretty sure that offering someone a job on a condition, say, them getting a haircut, does not make the employer liable if the barber accidentally takes off their ear. It's still likely to be seen that it was the individual's own choice to accept the job offer and the conditions, or refuse it. After that, liability would likely come down to those who made the vaccine, marketed it, and certified it for use. The employer would be at the bottom of a stack of different people who might bear liability. If the evidence shows it to be a safe vaccine, to the accepted scientific and medical standards, they have an incredibly strong case for saying they acted reasonably.
The thing is, 1 in 20 getting long covid and possibly being absent for months is serious concern for employers. And we're still in the dark about the long term prospects. Do some never recover? So you may as well ask - if an employer does not take reasonable precautions to protect their staff from COVID, are they liable for that?