I firmly believe that V/Vergil would absolutely not cheat on you.
As lonely as that poor man has been, I just cannot see it happening. He'd love and cherish you and never abandon you, after you get through his defensive wall, of course. I just do not see him seeking out another's company if he already has you.
And no, I'm not counting when he left LiR, he was a dumbass teenager who was blinded by his own thoughts and terrible past, and on top of that, I believe it was just a ONS
They care so much about each other. They resent each other. They trust each other. They don't trust each other. They don't want the other to ever leave their side. They feel like the other's presence is suffocating. They want the other to know everything about them. They can't let them know anything about them.