Vanessa and Ann are friends, thats why she isn't avoidable. She is MC's friend too, if you aren't blackmailing her. You can tell her to stop when Ann starts to get uncomfortable, besides Ann literally asked for her to help her. Both her and MC know how Vanessa is, nobody sexually assaulted anyone. Everyone knows Vanessa is very forward sexually, thats why Ann asked MC for Vanessa to help her with her own sex stuff.
Besides that, I think she was uncomfortable because MC was there, not necessarily because of what Vanessa was doing. She and her are friends, they've known each other for who knows how long whereas MC is the new guy, Ann gets nervous around him.