The vaccine isn't 100% effective.

The point of the vaccine isn't to make everybody bulletproof. It's to get the virus's R0 below 1 so it trends downwards into extinction. And we're on a timer between initial vaccine rollout and eradication since viruses can mutate rapidly given the opportunity. The fear is that a new strain can crop up, rendering the vaccine less effective.

Morons not getting vaccinated give the virus a vector to spread well enough to stay in circulation. And the longer it's in circulation, the less effective the vaccine becomes. So I may be vaccinated, but there's still a 7% chance some wingnut plague rat can get me sick. And that chance will only go up as the virus mutates.