vaccines, which great for stopping severe symptoms, don't prevent people from transmitting COVID, so if a place is full to capacity without social distancing I don't see that the lack of a few non-vaxxed people makes people significantly safer.
I think it depends on what a ‘few’ is, I guess. We know that two doses, even now and against delta, are preventing around half of infections (quite a few sources for this but the ONS is probably the best one as it looks at all infections and splits out cases from symptomatic cases).
Boosters look like they’ll send that through the roof, and the protection will last for longer than with two doses (early data from the trials so not authoritative or evidentiary.)
The data is still really mixed on secondary transmission (how likely a vaccinated infected person is to pass on to another person.) I don’t think it’s possible to be authoritative that vaccines either don’t reduce secondary transmission or that they do.
So if you’re seeking to reduce, not eliminate, transmission, restricting high risk events to only people who are half as likely to have the virus and half as likely to be able to catch it from someone who’s infected isn’t without merit, and is greatly strengthened as increasing numbers get boosted and the transmission risk falls even lower between the vaccinated.
But it depends on the policy objective.