This has consistently been very poorly reported in the press.
It is (obviously, when you think about it) much more difficult to study relative rates of transmission from people who have been vaccinated, than to study whether vaccinated people are getting sick themselves. So we don’t know to what extent vaccination reduces transmission, especially with new strains circulating. But it is certainly likely that a vaccine that is so successful at preventing disease will also reduce transmission significantly.
Careful scientists prudently saying we don’t know if X is true is consistently poorly reported to imply that it means we know X is false.
So your conclusion is right - it’s prudent for vaccinated people to keep wearing a mask for the time being. But that’s not because we have any reason to believe than vaccination is ineffective at reducing transmission. We just don’t have firm data yet.
This study just started: