Indeed, Meridian successfully converted the armored vest mod to png and it works. I thought converting from .gif to .png didn't work, because I had no luck with it before, but after seeing Meridian's response I tried a different converter and it does now
You can find the Armored Vest mod here (fixed for OSX):
https://openxcom.org/forum/index.php/topic,5737.0.htmlI also converted the Combat Uniform Armors mod and attached it to this thread.