For the donator capes, are they only usable while playing the aether 2, or can you use them in vanilla minecraft as well?
Do you mean, Usable when you don't have Aether II installed yet but you still donated?
Because the only way to access the Donator menu (where you will change your cape skin) is by having Aether II installed, and if you have so you are no longer playing vanilla minecraft.
This question was like... Check from chess. You can't realy do something without losing your king (the donator menu)