Yeah, that'll happen. The vaccine drastically lowers chance of getting infected, but it's not guaranteed. You can still get infected.
The delta variant is newer than the vaccine, and so the vaccines are slightly less effective against it. The real worrying part is if people start getting hospitalised -- not infected, not symptomatic, but majorly, mortally ill -- when infected with strains of COVID the vaccine was actually developed for. Then you've got a faulty vaccine.
