The UK did a large study of household transmission, large enough to include a statistically respectable number of breakthrough cases among vaccinated people.
The vaccinated people who never got infected of course didn't pass along an infection. Of those who had been vaccinated but still got infected, the chance of passing it along to a household member dropped by 38-49%.
Take the reduction in infections into account, and the estimate is that vaccinations cut the effective reproductive number by a factor of 6. The virus cannot afford that. Even in the early days in Wuhan with no precautions it was only 3-3.5.
We're still waiting for the results of the prospective study on college campuses but this strikes me as enough data to encourage vaccines to protect the unvaccinated.
I'm eager to see how the nasal spray vaccines turn out. Medicine is horribly unpredictable but it's reasonable to think that something stimulating the mucosal immune subsystem could be even more effective, maybe even much more.