I don't find it offensive at all, so please don't misunderstand. It's just, for right or wrong, my mind cannot separate "bro" from a certain mental imageone that is heavily male and, well, bro. I am not a "bro", and dammit, I don't want to be! *heh*
And I thinkpurposefully or notwhen a term like that is tossed around, it does to some degree create a feeling of gendering the group. There's never a shortage of examples of how women have trouble fitting into the gaming community, and if a female goes into a thread and sees "Vitabros" being tossed around and is called a "Vitabro", I absolutely think it can create a barrier for them mentally. Will some female gamers not care? Absolutelybut some also will.
Again, I don't at all think any of the members of NeoGAF's Vita threads want to in any way make Vita-owning females feel uncomfortable. I just think it's a case of a term being used by a majority who don't even think twice about it, but one which those in the minority might pay attention to.