A 90% reduction of infection means vaccinated individuals still get infected. That's what other countries have seen as seen.[1]

300 unvaccinated COVID-19 cases in current Singapore outbreak vs 78 vaccinated

If vaccinated people still get infected (even asymptomatically or at a lower rate) then they can transmit to the unvaccinated. There is no sterilizing immunity therefore the herd immunity threshold is likely much higher than 75%. Basically if you aren't vaccinated, you aren't protected.

[1]https://www.channelnewsasia.com/news/singapore/covid-19-vacc...