So many people completely misunderstood Vanessa's character. Typical adult game's progression with cold/bitchy character is to have them change and be good before finally having sex as the end goal. Vanessa is the opposite where the end goal is to have her character change for the better (i.e.: not being cold against the world) and having sex for her is simply a step towards reaching the goal because it's a compromise she's willing to do to test the waters before changing for the better in the future.
She clearly has trust issue so she treats everything as business transaction (even to the point of needing to pay her father back despite zero need to do so). You fulfill your part, I fulfill mine, and that's it. She doesn't really want to do more than that and doesn't expect anyone to do more than that either. That's why it's always the little (extra) things that MC did that she never asked for that won her over. MC expecting to stay in touch after the competition surprised her. MC remembering how she likes her coffee made her sure that she wanted him. But she's still unsure and she's uncomfortable with the idea that she's willing to let MC in to her life, she still has trust issues in general. That's why she insisted on MC making promise to her that she'd win the competition. Only after MC made the promise that she followed up with the invitation to her place. Her justification is "this is compensation for the promise" if something goes wrong.
Try that she may, her true feelings started leaking out. That's why she was the one who initiated the kiss when she said "just stop talking already" ... she knew what MC wanted to do and she knew she wanted the same thing. Then she followed up by inviting MC to her bedroom which MC responded in a joking manner. But she shut it down by saying no more jokes tonight, and even added "Please" to it. It's hard to keep up the cold mannerism when she really didn't want to do it at that time.
Her mask was up again after having sex, trying to test MC again whether it's back to just business now that they had sex. When MC asked how it was, she thought her answer would be to feed his ego. But again and again, MC showed himself to be genuine and what he wanted to know is really whether she enjoyed it or not. Not about him, but about her. With the mask slipping after his response, she tried to put it up again by changing the topic into "competition is not over yet, so focus" kind of talk. Yet, MC's response was to tell her that she could call him even if it isn't about the competition, if she just needs someone to talk to. When MC followed it up with how special the night it is for him, Vanessa clearly felt like her mask would be shattered right there and then. So she had to stop him before she lost it all.
I really enjoyed the relationship between MC and Vanessa, their journey has been fun, and I'm looking forward to see what will happen in the future updates. What will happen if they actually lost the competition? Unlikely to happen, but an interesting thought none the less.