They are effective. The vaccine stops delta between 70 and 90% of the time (depending on which vaccine and which study you look at). The "impossible to get herd immunity" scenario is within the error bars, but I wouldn't state that as a fact. On the contrary, it looks like NYC just reached it's Delta peak 2 or 3 days ago. This strongly implies herd immunity is still possible, but we need to wait and see if the trend holds.