The problem is not the vaccine, it is when you actual contract COVID-19 later on. If the vaccine sufficiently suppresses the symptoms, but doesn't actual prevent infection, there is a change you can spread the virus asymptomatically.
Assuming everybody got the vaccine in the same instant, which they would not.
> We should still shelter in place, but even turning active cases into carries would reduce loss of life (presuming we continue regular testing so people can KNOW if they are a carrier)
We've all seen how people are behaving right now: "Oh I'm young, forget about grandma". How do you think they'd behave knowing that they've been vaccinated? It's important to consider the sociological conditions in which an intervention is deployed. Unfortunately, our present society has a very large number of self-centred individuals.
The more people take the vaccine, the less society bothers to social distance and other low-tech things, the more dangerous not taking the vaccine becomes.