The people mentioned who believe that vaccinated people will "shed" the virus onto the unvaccinated have heard it said that "Unsymptomatic vaccinated people may pass it on" without adding the few words left out - " If they do catch it, or had already caught it before vaccination, but don't show any symptoms vaccinated people may pass it on"
There is a kind of logic from the facts to the fantasy, but they don't seem to realise that the virus is NOT a live one, which could be reproducing in the body, so there is nothing to shed.
If you get a chance, tell them the bit that has not been made clear in the statement, and add that the vaccination doesn't involve the live virus, so cannot be passed on unless someone has actually caught the real virus. That is if they will listen to you - there are none so deaf as those who won't hear.