What makes you think he is an INXP and not an ISFP?
http://thecolbertreport.cc.com/videos/gds7n9/justin-vernon
That interview was the first time I had seen him and he came off very much as INP in his mannerisms and his short, concise, and sometimes unclear language. He really reminds me of my sister who's INFP, or even how I might be in a similar situation. In college he studied philosophy, comparative religion, and women's studies before studying music, which really seems Ne driven to me.
My mom is an ISFP, and so is a good friend of mine (a singer). Neither of them would ever have considered similar types of study, much too abstract and beyond what's required for every day life. The ISFP singer I knew was very talented, but disliked music theory quite a bit, and preferred to learn in a more hands on, by ear approach. She was a natural performer and her lyrics were much more direct, with little to no hidden meaning or obscurity, though very image based.
My career is music (performance and recording), and I can very much relate to how he approaches his lyrics and song structuring, as well as his focus on emotion. It's much easier for me to express emotion through music or an instrument than in a conversation. It just requires using a bit of focus to allow Ti to relax, which can be learned. In fact, playing an instrument is one of the only times I get to take a break from Ti and mostly just feel. Its rather freeing and meditative, similar to how it seems for Justin. If I had to put money on it, I'd probably go with INFP though.
EDIT:
But he does come off as very INTP in this interview from Austin City Limits:
Bon Iver Austin City Limits Interview on Vimeo
The way he talks about his songwriting, his concept of music, the brain, voice, and performance, is very similar to my own way of approaching music. That and his low-energy, somewhat monotone way of speaking.
Actually, after re-watching this, I'm leaning toward INTP.