> In my experience they do work exactly that way, you take a vaccine and then you are far less likely to get the disease. It’s true that you cannot reach herd imunity or reduce the spread until you get a large enough population

The problem is there are some people who have real actual reasons they can't take the vaccine. They need the population to reach herd immunity for them to be protected. If other people opt out and there's no herd immunity, they die.