1) My understanding is there vaccine won't necessarily prevent infection, but should prevent severe cases. Is that correct?
2) if correct.... suppose a vaccinated person contracts a mild case, then they're still capable of passing it on to others, right?
I can see how these are confusing. Many answers in science aren't 100% and then we are only really able to comment on what the data actually shows. The vaccine trials looked at symptomatic covid and severe covid as endpoints of their study. So both vaccines had protection against symptomatic covid and that is where the 94-95% comes from. There weren't enough people that got severe covid for Pfizer to say with statistical significance that severe covid was prevented, while there were enough cases for the moderna vaccine. (This is all due to the number of people within the placebo group that got severe covid, and likely has nothing to do with the actual vaccines. Other than they both are likely going to achieve that with time. I say all of that, to state the vaccine trials did not really look at asymptotic cases. (moderna a bit at 2nd shot, but wasn't really part of the study design - and again, was promising, but not high enough numbers to make claims). All this is why we don't have statements on asymptomatic infection in the vaccinated population. To me it stands to reason, and makes sense that there could be some underlying infection going on, however, it is going to be at a much reduced rate, No data on transmission, but likewise, the reasonable assumption is that it is going to be limited as well. So while technically, and given the number of people and their varied immune responses, there is going to be some mild, underlying cases of covid still passed around in the vaccinated population, they are going to have reduced transmission, decreased disease severity, and are going to be extreme outliers, (not frequent at all) and with the total numbers of illness inevitably dropping, the likelihood of this rare event is going to decrease as well.