When you create a custom field via the menu Display>Custom fields, you can choose if you want it to pertain to the address or to the user.
If you choose to create a custom field for the user, it will actually be editable in the user profile edition page of the user like you want.
However, the address_vat field is a default field because some of the functions of HikaShop rely on it to work properly. If you don't want to use it and create a user field for it, you can, but the tax system won't be able to use it to know if the user has a valid vat number or not (for Europe), and you won't be able to add it in the address_template view file so that it displays below the address of the user in the invoice of the users (like it's commonly done).