There are two vacations, so far. Near the beginning of the game where Anne really spreads her...legs. And the Anniversary vacation, currently at the end of the update (around Day 12-14). Assuming that you are asking if Anne cheats during the vacation scenes and if you can avoid it. The easy answer is no, you cannot avoid her cheating. The sticky answer is, as the MC, do you consider it cheating if she tells you about it afterward...occasionally? In my opinion, Anne cheats the whole time. It is likely just sloppy writing that makes it that way.
The agreement the couple has is that they will play together, or that she will tell him about it afterward. There is some text that even makes it sound like she will not sleep with anyone without the MC knowing and approving. After all, the point of her sleeping around in the first place is that it is the MC's kink for her to sleep with other men while he watches. Twice, during the course of the game so far, she has said that the only reason she does any of it is for HIS enjoyment and because she loves him. That is all bullshit as she sleeps with far more people than she ever tells the MC, she has relationships with these men (not just sex), and she all but says she is in love with three of the guys she is fucking.
Now, you could have a lesser definition of "cheating" and be possibly happy with the fact that she is sleeping with everyone and occasionally tells you about it. Even if that is the case, while on the first vacation Anne will have sex with at least one (based on your choices) man that she does not tell the MC about. If you did not choose to follow her, there are more cases where she sleeps with someone and does not share it. On the second vacation, you have very little choice at this point. She is going to do what she wants, with or without you.
Sidenote: if you want her to cheat...This game is FOR YOU! She will cheat from the moment you let her sleep with Ryan or the dad. That's what this MC gets for being a total douche nugget and not tightening up on the chain!
TL;DR...She cheats no matter what.