Sorry if this is stating the obvious, but the vaccine, while it won't give you 100% protection from catching the virus, it will make it significantly less likely that you'll get it and then, if you do, make it less likely that you'll pass it on. So, if that can be replicated widely within the general population, it will make it much harder for the disease to spread. So those who get it won't pass it on and eventually there will be much less of it around, so it will be even less likely that people will get it. That's what herd immunity is. If only a small proportion of the population can catch and transmit a disease, it's unlikely to spread widely. The disease essentially keeps coming up against dead ends, rather than finding multiple potential pathways to spread.