Why didn't Victor recognize Lauren?

Because it wasn't really a relationship as far as Victor was concerned. He, as you see in the movie, just goes from girl to girl. Seeing that Lauren didn't even have sex with him, he quickly moved on and forget her.

Lauren was more likely to remember Victor because she only really had a relationship with Paul, sex with the teacher, and a crush on Sean. It's just a numbers game, really.

The way i saw it he was so stoned or drunk or whatever that probably played a part. Plus as we learn in the movie he had been through an awful lot in England and then Netherlands so that also sort of explains why he forgot, and he didn't "love" her like she loved him.

They were never together as a couple. She was just another conquest, probably just a one-nighter, any girl at any party.

As others have said, she was not important to him. He was significant to her, but to him she was just one of the many.

In the NOVEL, Lauren and Victor's "relationship" fares a bit better ... But just barely. Victor still has no idea who the hell she is, and refers to her as "Laura" at one point, if I'm not mistaken. At the end of the novel - right before Christmas break begins - he agrees to go home with Lauren to Manhattan because he has no money.

I think her and Victor met, and probably had a night of fumbling about. He probably fed her lines to get something out of her and Lauren took it too heart and probably believed all the crap Victor was feeding her. Victor seems to like sleep around and probably quickly forgets the women he sleeps with it, look at his trip to Europe he probably fumbled about or slept with a dozen women. Throw in the girls he slept with in college and Victor just simply forget about Lauren, although he mentioned her during the whole Europe trip as another girl reminded him of her. I'm sure he probably liked her for a short time, seen as she didn't have sex with him but he quickly forgets her.