I think, for me at least, it's the challenge. Not every character has to be a shy innocent virgin that fawns over the MC like most games have. It's nice to have a wide diversity of characters (like this game has).
Vanessa is a bitch. Plain and simple. But look at her background and lifestyle, and you see why. That's just who she is. The whole point of even needing to interact with her, is for our thesis. And she's a different character perfect for that. Best part about her, is she doesn't like the MC. So it's like, "challenge accepted" lol. It's fun trying to win her over, and be flirty or a joker to her and gauge her reactions. And especially in this latest update, you learn that you and her are not that different after all. You've both been through similar situations in your past, and that helps with getting her to warm up to you.
So yeah, while she may be a bitch and such, I still find her the most fun and intriguing. Though Harlow is my other main squeeze (complete opposites!) lol.