Let's say 100% of folks who are vaccinated (or acquired natural immunity) get infected (we may actually achieve a high percentage, eventually). That's not the measure of a vaccine's failure. The odds of not getting seriously ill or dying are dramatically reduced. That's a result of protection vs efficacy.
https://www.msn.com/en-us/health/medical/breakthrough-covid-19-cases-expected-to-become-more-common-in-coming-months/ar-AAOH52V?ocid=BingNewsSearch