Dan's like, "Hold that thought, I have to go get railed and bitched at by Vanessa, who shoehorned me into an impossible position and was incredibly pushy, and now is angry that I didn't immediately clear my schedule for her rude ass." Serena is cool with that. He asks what the hell Vanessa's doing there, and she's all, "I'm writing an American History paper... Oh, wait. That was supposed to be you." Snotty beast. "When did we start lying to each other?" Um, right around the time you got aggressive and weird? You out-Danned Dan. The King Of All Danship got Dan'd, and he didn't know how to get out from under it. We know how you feel. After a babbling dry run at explanation, he explains: "...What was I going to say? That I was renting a tuxedo to attend a masked ball without a mask, or an invitation, to go get a girl who not only has a mask and an invitation, but another date?" Actually, that's exactly what you should have said. Vanessa would understand that. Vanessa would be like, "Why didn't you just break into her house?"
Vanessa says that at least the truth would be honest, but then S watches as Dan admits that Vanessa's too-cool-for-school propaganda still carries some weight with his stupid ass, because he was embarrassed to tell Vanessa all of that because she disapproves of...masked balls. Proving that Vanessa is a halfwit, but also: Do not apologize for your life! They never listen! (They are in the TV!) So Dan's like, "So...you followed me here to prove I was lying?" Which is lame and true, but not as lame or true as the thing she says next, in a sarcastic tone that totally underscores how lame and true it actually is: "Yes, Dan, that's why I'm here. I stalked you because I am just so obsessed with you. Wait. Don't stop there. You're the whole reason I came back to New York, 'cause I just couldn't stop thinking about you." Which, okay, I'm wishy-washy, because that kind of hurt my heart a little bit for the jerk, because that's exactly what she wanted to say at the beginning of the night. And it's a no-takebacks kind of thing to say, but now she has said it meanly and hatefully, so she can't ever say it again for real, so she kind of just Vanessa'd herself pretty impressively, and that's sad for her, because no matter what happens, the true thing is off the table forever. So the lesson is, never say anything honest to the people you're dating, because they will screw you with it later. At least, that is what I take away from this scene.