As for being the butt of the joke - it's actually quite tame to the shit Vincent gets from Johnny and Orlando. They don't bully her, nor is joke on Erica when she sleeps with Toby. You actually laugh at Toby's predicament because everyone throws hints his way, but he's too naive to get what they're talking about. Not once do they say, "HA! HA! YOU FUCKED OUR FRIEND WHO USED TO BE A GUY!" In fact, when it happens, they avoid the subject.
And honestly, it would be an awkward subject for them to discuss because of the history involved. At the end of the day, she's confident in her own skin - unlike her schoolfriends. That's sort of the point.