The answer is that variants will continue to spawn as long as we have enough hosts.
Vaccination dampers the ability of the virus to spread and mutate. In vaccinated people, the virus has a harder time getting a foothold.
If you're vaccinated, you'll be exposed to the virus the same way unvaccinated people are exposed. But your immune system will respond differently.
In unvaccinated people, the virus has more opportunity to flourish & mutate.
The Delta variant is not so different from the alpha. So the vaccines work almost as well against Delta. Not *as well* but almost.
If the unvaccinated population remains at a sufficiently high level, there will be enough places for the virus to thrive and mutate into something your vaccinated immune system won't recognize.
Then your immunization becomes useless.