Sorry to have inadvertently started another 6e armor debate thread. That was not my goal. In fact, part of the reason for my question was so that I could work out a starting point get that particular mechanic out of the 6e rulebase. That's part of why my question was not tied to any given edition.
Regarding using a vest as a baseline to stop a pistol bullet, that seems like a reasonable start point, but would need to be not 100% effective due to the coverage area being smaller.
I suppose I could also think of it that way for things like a lined coat, which might not have any better stopping power, just greater coverage, and thus better overall protection. So a lined coat should be more likely to stop the bullet, even if conceptually it has no better density that a vest. An armor jacket by comparison is a heavier armor, harder to breach, but with a little less coverage. So thus you still get a higher rating with that overall.
Playtesting to get final numbers right is always needed, but
I'm looking to divine a theoretical baseline, in-universe. Does a vest compete with a pistol? The answer of meeting expectations is a good one to consider. With the thoughts above tho, I think the lined cost is my new baseline armor balance point.