Vaccinated people getting infected doesn't mean it's ineffective, and that goes for vaccines in general as complete immunity in vaccines is not as common as people think, it's nice as an ultimate goal but for the most part you aren't going to be invincible, but your existing defences will be reinforced and have early warning so the benefits of vaccination still monumentally outweigh the alternative.
The fact they're vaccinated means on average it won't be as bad or symptomatic for them, they'll recover faster and better and the virus is less likely to spread further from one person to another.
There's talk of a 4th wave being imminent here, but that it's the unvaccinated who are in the most trouble when it happens.