I want to change my minecraft screen name, but I don't know what will happen to my res, vault, or inventory. please tell me what would happen to my stuff if I change my username.
Your res, vault, and inventory will be fine Everything stays the same. And I believe if you have shop signs those would automatically be switched to your new name. That's from what I've heard, not too sure about that one though.
There is one thing that won't change, you player heads, those will still display your old name, nor will your skin change of those heads (you can change you skin to something silly and farm a few of them before you change your name, it can be nice to have some mini-blocks)
Also keep in mind that this trick won't be usable to "hide" yourself. After you changed your name players can still use /p <your old name> and they will then get to see both your new (current) name as well as your name history (your previous names).