If the follower is dead, then you should be allowed to get a new one... else you'll be advised to release your current follower first (which has to be done in person). Not great game design as I've complained before, particularly given the propensity for them to disappear.
I actually just found mine again today in the wild, seemingly by chance, after she was missing for... many many hours (and weeks in game) of play. She was engaged with a giant, who fortunately wasn't able to kill her. I wonder if her attempt to return to me or her place of origin resulted in her encountering the giant, and since she isn't able to be killed by it, she's basically locked into permanent combat with it. In other words, the giant hits until she crouches, then she recovers and it hits her again... Until I intervened and killed the giant. Or if the game doesn't trigger events away from my character, then maybe we encountered the giant together weeks ago and I simply left her... she'll often fall behind during travel and when I think to look for her, I double back and find that something has caught up with her.
Regardless, followers can be a pain!
PS. Anyone know if you can marry Vex? I think I'm in love!
