I know she only wears armor that has a better armor rating than her default armor, but I've been trying to put her in regular vampire armor, just for a change of appearance. I've even fortified the armor to legendary, even epic status, enchanted it and still nothing. Am I missing something or doing something wrong? I'm also on the PS3, so no modding of any sort is an option.

Try giving her Dragonscale or glass
Tell her to [censored] off
Paralyze her
Press Pickpocket on her straight after the paralysis effect stops
Take her Royal Vampire Armour (leave boots)
Now she will be naked and you can make her wear anything.

I tried that, but the Royal Armor didn't show up in her inventory, even in the pickpocket menu. But then I discovered she was gonna wear my desired armor anyway after giving it to her. I just had to leave the room/screen for her to actually change it. So problem solved.