I feel like a couple of scenarios could come of the vaccine effort:

A variant evolves that the vaccine induced immune response is so useless at stopping that it that if you've been vaccinated itis actually no different than having no immune system at all when infected by that variant, and all of these vaxxed people end up suffering sever Covid and have to be treated with effective antivirals or they die

Or, a variant originates in the vaccinated population that they have immunit to, but it is super deadly to unvaccinated people so the un vaccinated have to get vaccinated to survive.

In either of those cases, once it's approved they can call emergency, go on a serious lock down, but this time they come door to door with the vaccine.