What turned him off was the lie! Why pretend to have big breasts if you don't? The Seeding Ojisan will seed you either way, why must you hurt his feelings by betraying his trust with lies?!
I don't exactly agree. From what Zhul said, Vivian was actually losing all of her matches until her new sponsor bought her and started equipping her with magical items.
Mikogami is special because he's able to win while being weak and having no sponsor, while Vivian only wins because a rich guy...
We all know the real reason the so-called childhood friend is getting in the way is how she recognized Naoto and Kenji have more sexual tension between them than any woman can ever hope to have.
She enjoys it. Honestly, for as much as Hajime complains, he always ends up going along with her degeneracy because he likes her more than he wants to admit.