Regardless of whether the rest of your theory is accurate or not, if that is Vinny at the party, then it's almost a certainty that he did it. He's one of the few people who would have access to the DIK cluck acount. My gut always told me it was Nick, since he's still not entirely happy that Vinny left, but I never really considered that Vinny was even there. I feel like Vinny is going to be our main antagonist, so it fits that he'd be fucking the DIKs over from behind the scenes before he gets formally introduce outside of a flashback.
I have already written two, three times, so that I am divided in two reference to Vinny. I can't really explain it, but I figure so Vinny doesn't want to hurt the DIKs per se, but explicitly Tommy and maybe Quinn.
In EP9 yes Quinn mentioned so things have been perfect for her and Tommy since Vinny left. Well, more perfect for Quinn, as Tommy noted. Something happened between the 3 of them.
Because of the incident in EP4, which could have been Vinny, Vinny can't show up at the DIKs so easily in EP9. That's why Vinny brought the gun. Self-protection from angry DIKs who know he dropped off the Cathy Cluck and may also be responsible for the destruction of the mansion.
The only question is why Vinny would risk coming to DIK Mansion in EP9. Yes, the whole Pink Rose Girls thing and Tommy seem to be the reason, but I have a feeling so it's just Vinny's excuse for another reason because he could have sorted it all out off campus. But a Halloween party is perfect for getting into the DIK Mansion undetected. Sure enough, Quinn recognizes Vinny by his voice and he brings up the flimsy excuse about the drugs and the Pink Girls. That was just an excuse by Vinny.
But Vinny only shows his face to the MC voluntarily. Yes, Tommy sees Vinny's face too, but Tommy only joins when Vinny had already taken off his mask. Vinny wants to face the MC face to face. I think so the Quinn and Tommy are hiding something from the MC about the MC and Vinny. I don't know if it's kinship, love interest or ex girls but something is being hidden from the MC and Quinn as well as Tommy know what it is. Both of them show concern on the roof because of the clash between Vinny and the MC.
On the other hand, Vinny can be just an asshole. That's why I'm split on Vinny, because there are clues for both possibilities.