Well, while we don't know the whole ins and outs of their business relationship, at the moment we can only speculate. Just by following a healthy logic:
1. Vinny is quite dangerous, because from a former party-goer and a life-waster, he turned into a criminal and a complete scumbag. This is not a local junkie who sells weed in the yard. And we know perfectly well that Vinny buys more than just weed from Buddy.
2. Cowardice has absolutely nothing to do with it. Criminal activities is not a battle of knights and concepts of honor don't work here. MC literally did nothing to him, just retorted (more than justified), but Vinny was the first to go into conflict, and even if MC chooses a Chick option, he still threatens him with a gun. So Lily and Quinn aren't safe either. Apparently, Lily had known him for at least a year and he was her first dealer. I don't think her words about "he shouldn't know about it" are an empty sound or forcing the situation for the sake of drama. She must know him well enough to have a reason to say so. Quinn also know Vinny quite well, while they spent about a year together while he was still in college and DIKs. Quinn is concerned that he threatened MC with a gun, but her reaction is not "wow" or "how can this be", bc she mostly expected such behavior from him.
3. Let Vinny deal with Quinn? And now she have to put her over to him? This is a nasty and vile act. Quinn didn't want this herself, Lily was interested in this in the first place when she found out that Quinn's was selling shit cheaper and wanted to buy from her and drop Vinny, fucking him back in such way. Apparently, Quinn told her that they get shit from the same source (from Buddy), which is why Lily tells her friends in such detail exactly how Vinny fooled them and that there is no need to worry about the quality, because it's the same, only cheaper.
4. The real source of the problems is Tommy. He was the one who decided to drag Vinny back to where he left from, because of harboring a grudge against Quinn. He also very badly framed the MC, by presenting the really dangerous Vinny as a kind of freak and a fool who was just fucking around with a dummy gun. Although he had no choice, if he had told the truth, MC would probably have called the cops for real, and that would have been the end of DIKs, HOTs and many other characters.
5. In the game, de facto, at least 5 antagonists are clearly visible. Stefan, Vinnie, Buddy, Mick, Quinn. It's still unclear how Quinn's character will develop and whether there are any choices we could make to guide her down one path or another, or whether her fate is already sealed in the plot.
Mick:
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I'm impressed with such logic, seriously
Stupid idiot with a gun or "smart" criminal with a gun, it doesn't make sense when you get shot. They WAS dangerous, while it's you(MC), who was really stupid. Or MC just happened to be in the wrong place at the wrong time, as it usually happens. Vinny is clearly don't look like a stupid person. Impulsive or reckless one - maybe, but not an idiot. Even he is walking with a gun on him, I doubt he was ready to shot MC in DIKs mansion, next to a bunch of witnesses. He, as well, don't make an impression of a person, who want to end his life in a prison, at least he accepts the risks by doing what he do, but I doubt that he want to fuck up and being arrested for such a stupid reason, like, shooting to somebody in a public place crowded with people. But even if MC called for police, as he wanted to, I doubt that they would able to catch Vinny that time. He did it to threaten and maybe warn MC to not fuck with him. If we will have another scene with a gun in the future, I think it may happen not on campus, but somewhere else. Maybe on Quinn path, she could be the one to get MC into trouble, she asked MC to be her bodyguard. And we know that they have to visit Buddy's lair, it's a place where Vinny also lurking around from time to time. On MGs branches it's probably could be a Tommy. It's hard to guess now, how it could be implemented, but why not? On MGs branches Quinn have nothing with MC, especially when it's J&M or Jill ones. But I don't think that future dealing with Vinny will be exclusive to Quinn path.
Dog - this is both a mockery and a warning.
"little puppy" - if MC tries to be as correct as possible (Chick)
"dog that barks but doesn't bite" - if MC is just rude to him and asks him to fuck off (Neutral)
"rabid dog" - if MC punches him and says that rabid dogs get put down (DIK)
Zoey warns MC after he punching Oscar, that it may end bad for him sometime. Even considering that this is a game, but sometimes it is useful to pay attention to such hints from the author, because they are definitely made to warn the player. The same thing happens when MC reflects with an inner voice about relationships and other things.