I'd think that a good portion of the reason why vests don't help much is because most car headlights really don't shine that high. Headlight patterns have been made with a good horizontal cutoff for quite a few years now (of course that assumes that they're aimed correctly, but let's be honest -- MOST vehicles out there DO have correctly-aimed headlights), and they really don't illuminate very much above level -- which is maybe knee-level for cyclists.

Do an image search via Google to see what I mean; use "headlight cutoff" and you'll get plenty of results.

FWIW, I've been able to read street signs much more clearly with a helmet light or a bright handlebar light than I have with my car's headlights. Not because the bike lights are any brighter -- which they're not -- but because they have more glare.