A piece of steel to protect the chest used to be pretty expensive, as well as well made weapons. So, if you consider this fact, it would be pretty common both lowborn male and female warriors and adventurers to have a few parts of armor to protect themselves, while a noble and rich people would be able to afford a full plate armor. Also, armors are made of many diferent layers of diferent materials.
This is a fantasy game, but that concept would explain the "bikini armors"...
maybe, if characters could have an extra slot to equip clothes of diferent kinds that would show beneth the armor... so you could have cloth and chainmails under the armor or take it out to show the female character's atributes.
A good example of that idea is Neverwinter (online). I don't know if its something easy and viable to make, but surely would please both publics.
Bikini armour shows bare skin, especially cleavage and the belly. Both of these regions would be protected at least marginally by even the most destitute warrior, if only by pieces of leather or cloth. Most vital organs are in that region, so why would anyone with any kind of sense leave them unprotected? And yes, people of the time probably would've known that a hit in the stomach or chest was bad.
This is probably more fantasy than reality, but a poor warrior would probably scavenge for bits he could use and at least bolt some metal parts on a vest. Bare skin is just a complete no-go in any kind of setting that doesn't want to rely on titillation.
