Nah, I don't think it's about the purely sexual aspect of dom/sub roleplay at all. As I see it, in this department the MC is as inexperienced as he is curious to discover what he likes. All of Bella's attempts to somehow offend him and make fun of his inexperience simply break down due to his complete ignorance.
And in the end, she even likes it, remember their dialogue when Claire spilled her box of toys. I get the impression that her aggressiveness rather turns the MC on, and he doesn't mind playing a bit of a passive role, he just regrets that he can't give her what she wants right now.
But with Vanessa it’s a completely different story; for her, sex is a tool to submit a man, a way to assert her dominance over a partner not only in bed, but also in life. The MC understands this very well, and Vanessa does not hide her motives at all. She makes it clear that she made Mario wear the buttplug to show him who's in charge. I doubt that Nika would agree to such kind of relationship.