So if 30% of the population of your community has Covid19, then 0.3X0.05=0.015 which I believe means that if a fully vaccinated person goes out and about in that community without masks, social distancing etc, then the chance of catching Covid19 for that vaccinated person is 1.5%. Is that correct?
Over the entirety of the pandemic, but that is a bit of an after the fact thing and also a bit hokey (see below), since you will know the net attack rate for the entire pandemic (though not of only the unvaccinated in any easy way) only after it is all done, and the attack rate can change wildly on the way to the end of the pandemic. It is more useful to get a feel for what is my chance of getting infected today.
This more useful use as far as I understand it, is to see what is the daily case rate per something like 100K is, which gives a you a good estimate of the attack rate. That multiplied by (100-efficacy)/100 (i.e. 95% efficacy gives the multiplier of 0.05) gives you what the infection rate would be for an otherwise unprotected vaccinated person.
So for example today the case rate in my county is 6.5/100K, so the estimated daily case rate for an unprotected vaccinated person who has a 95% efficacy vaccine would be 0.325/100K or something like 0.0003%. This is the reason that CDC has relaxed the mask thing for the vaccinated, specially with an R-0 down at 0.76..
If we use the cumulative cases so far as a proportion of the population in our case it is about 10%, so yes your computation would give 0.5%, but that is a synthetic number, in the sense that since the vaccine was not available through the entire period it is hard to imagine how somone could have been vaccinated at all over that period, and if it was available that would skew the number significantly making it not great a proxy for the unvaccinated. That is becoming a significant issue even with the daily number as more people get vaccinated. So using it as a proxy for unvaccinated will tend to give a lower than actual estimate of infection rate for the vaccinated.
Does that make sense? I don't claim to be an expert but this line of thinking seems to be an useful way of making practical use of the jumble of numbers we get every day.