As Mark pointed out, terminating an employee who refuses to get vaccinated wouldn't be "slippery" at all; there's ample legal precedent. Employers have always been able to mandate compliance with safety rules and procedures as a condition of continued employment.
And yeah, I would imagine there will be lawsuits when an employee who is fired for noncompliance bleats about his religious freedumbs or his paralyzing fear of adverse reactions. As far as someone getting the shot under duress and suffering a serious or lethal reaction--well, how many times has that happened? What, one person out of 75 million vaccinated developed lethal blood clots? It's not really a potential issue.
I would frame it this way. You work among other people, you assume more responsibility than you would have if you lived in a dark hole in the ground and never came out. There's an implicit social contract that you will not put other people with whom you come in contact in danger. We agree to obey traffic laws, not assault others or put them at risk of injury, not play with explosives or toxic chemicals, etc. I don't see how agreeing to not be a potential lethal virus carrier is any different.