I think you're right, and an additional factor is that the vaccine is tangible and certain where the virus is invisible and may or may not be present in any given situation (and in unknown quantity).

At its core it may be more that people can't think past binary logic. Either something works or it doesn't work. Either something is perfectly safe or it is too risky. But that doesn't address why they don't apply the same logic to the virus itself (although there are some people on the flip side who still do).