Because, despite Greg can be an obnoxious asshole, he's still leagues above Vaush and Shoe knows it. Not defending Greg, but he's trying to decow himself and he's now in a much better position he was a year ago. Vaush is never going to move from the place he's now. The man has no ambitions, no motivation, anything. And it's not because he's an obese blob, it's because his mind is too as well.
Also, it's all about image for Shoe. She's never going to date a man who's universally mocked for having a micropenis. That's not very chad of him.