I 've met an issue trying to use shift code in https://borderlands.com/en-US/vip/. It’s unable for me to add my Steam profile to vip account, after my successful logging to steam I was logged out on vip page .Pop up window with logging shows message in red that something went wrong as my username&login info fields are blank, than it’s closed by itself with no chances to fill info and sign in. Then it threw me back to default borderlands.com/en-US/vip unlogged page. It was reproduced several times with the same steps and result.
Shift page still requires to add steam profile to use shift codes with vip profile.