At first Vegas thinks that Pete is only laughing to mock him or to act cool, so he shows him the sparks. But after he hurts him several times and Pete still laughs, he is intrigued. For one, Vegas is impressed by Pete’s endurance and ability to smile no matter what and second, Vegas interprets Pete’s strength as a challenge - which it was. You can see Vegas’ interest in Pete changing the moment right before he asks him ‘One more time?’ He wants to test him and see how far he can go. He wants to know how much Pete can handle.
We only see Vegas’ reflection because we only see Vegas as whatever he chooses to be for a person to manipulate them. He is only a reflection, not his true self. He stands in front of the mirror as if he decides what to look like and what mask he picks for the next person.
The mirror also symbolises that Vegas is a two-faced person.
