The short answer from an immunity and pulmonary medicine physician

Q: Once a  COVID-19 vaccine is available, how much of the population will need to be vaccinated for the pandemic to end?

A: A vaccinated person acts as a barrier to slow and prevent the virus from continuing to spread. The ultimate end goal is to get as many people vaccinated as possible so that more people are protected. The entire population benefits from a high vaccination coverage. 

Once a population reaches a point of collective immunity where the disease is no longer likely to spread, it reaches the herd immunity threshold (HIT). The estimate for COVID-19 is that 50 to 80% of the population will need to be vaccinated to reach the herd immunity threshold. (In contrast, the flu needs between 33 and 44% vaccinated to reach the herd immunity threshold.)