And a second unvaccinated coworker is also someone that has chosen not to be vaccinated, right? (Obviously there may be some individuals who can't get the vaccine, but that should be a relatively rare circumstance, and could have policies around dealing with it.)
Sort of; we don't (and won't) entirely know.
With the delta variant becoming more common, it's seeming like vaccinated people might still be able to transmit the disease, even if they don't become infected themselves. There's also the matter of asymptomatic infection: remember that the 95% efficacy numbers reported by Pfizer and Moderna (for example) are about symptomatic infection; during their clinical trials, they only tested people if they showed symptoms.
And if a vaccinated person is infected but asymptomatic, they may never even know about it. Technically we may or may not call that a "breakthrough infection", but that person could still presumably infect an unvaccinated person, and that's the thing that matters.
> Obviously there may be some individuals who can't get the vaccine, but that should be a relatively rare circumstance, and could have policies around dealing with it.
Right, and having people around them who are unvaccinated by choice is an unnecessary risk to those who can't be vaccinated.