Are you going to address the stacking of vulnerabilities, Brumm? For example, If you have greater seal defender's heart and greater atlantian golem's heart in your inventory, you should get 10 % acid vulnerability and 20 % cold vulnerability in total. However, logging in a second time (or switching back to a server you were previously on) makes them double, resulting in 20 % acid vulnerability and 40 % cold vulnerability in total.
I tested with two characters. The first one had 85 % fire immunity from items. Iron body would have previously caused the character to become vulnerable to fire but now the character correctly resisted 100 % of the fire damage. The character also had 100 % (105 % theorectically) immunity to electricity (not synthetic) from items and casting Energy Immunity didn't change it. Shadow shield in addition to 50 % immunity to negative energy from items resulted in 100 % immunity.
Another character had 70 % immunity to acid, cold and electrical. I applied energy immunity to cold and electricity and the immunity worked. I also tried a combination of energy immunity to acid and iron body and it resulted in 100 % immunity to acid.