The idea that vaccinated people are an island unconcerned with the fate of the un-vaccinated or the spread of the disease breaks down in two places:
1. You can still die from it if you have the shot, you're just substantially less likely to.
2. Eventually, unless they're stopped, random mutations will create a virus that the vaccine doesn't help with at all.
Honestly, the fact that the virus is running around replicating in a way that's constantly exposed to anti-vaccination selection pressure (both from trying to jump out of the un-vaccinated reservoir to vaccinated people and replicating to a limited extent in the presence of vaccine-primed immune systems) is in some ways a worst-case-scenario. It would be hard to intentionally design a vaccine escape guided evolution experiment more effective than what we've created in the world today.