I absolutely agree that armor is a worthwhile purchase - that's why buy it.
But the conversation had shifted from the pros/cons of having armor to ways that you can overcome other people wearing it.
I think it's worth having, but few enough people do that I don't feel it's necessary for most of us to spend a lot of time worrying about what to do if you're attacked by other people wearing it. Even if it wouldn't kill them I'd think getting punched in the vest by a few bullets would be enough to make most people stop what they're doing and look for cover.
si vis pacem para bellum