A vag-bro is a female who behaves in a fashion that is typically socially acceptable around bros or other high school or college age males. Not to the point of butch, but just enough to be accepted as "one of the boys". She isn't necessarily lesbian, so many men who do not want to deal with the usual high maintenance girlfriend will often gravitate toward vag-bros for romantic affection.
Bro 1: Oh man, who's that hot chick by the stereo? She seems so...chill.
Bro 2: Yeah. I heard she's a total vag-bro. I'm gonna go try and get her number. Even if we don't end up dating, she'll be awesome to hang out with.