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
While Tessa's full intentions are yet to be revealed, she is still going through a lot... and it's illustrated through her body language. While bitter about what the AbsoluteSolver supposedly did to Earth, she appears reluctant to break it down to N and V of what she deems necessary.
When telling N about needing to either save the universe or Uzi, she briefly looks away in shame. I noticed that her voice and holding N's hand are in a gentle way, which is sweet.
She knew that killing Uzi would take a huge toll on N due to how close the two became. That being said, she lets N hug Uzi as if wanting to give him the remaining time to have her before making the choice.
After escaping from Alice and the Sentinels, V tells off Tessa for putting all of them in danger. We get a lingering camera shot of Tessa holding onto her bleeding arm, which is the same way she did around her mother (who chained her wrist in the past as a form of punishment). This time, V had a right to be angry at Tessa for her assumption of controlling the Sentinels. But it's possible that the guilt was starting to sink in, especially when Tessa solemnly confessed that she hasn't been honest with V yet.