It depends which study you read. According to the article below, some estimates using viral load as the determining factor in transmissibility may be overly-pessimistic.Research is emerging that transmission levels are unaffected by vaccine status or at least the reduction is minimal.
For one thing, they often look only at the chances of onward transmission if a vaccinated person is infected, without factoring in that they're less likely to become infected in the first place.
How much less likely are you to spread covid-19 if you're vaccinated?
While there is still some uncertainty involved, as it states:
De Gier says they cannot calculate the full reduction in transmission due to vaccination, because they don’t know exactly how much vaccination reduces the risk of infection. But even assuming vaccination only halves the risk of infection, this would still imply that vaccines reduce transmission by more than 80 per cent overall.