I agree with you partially, but I don't think Vinny is too stupid. We have no information indicating that he is stupid beside his wild behavior in the past (as part of the DIKs), but that was probably due to his wild nature, not stupidity. I feel like right now Vinny is the true antagonist. In episode 6 Buddy says Vinny is doing really good with his drug business (better than Quinn) and both Quinn and Tommy are visibly afraid of Vinny.
Vinny is great material for the main antagonist because he is basically the antithesis of the MC. They both quickly gain much power inside the DIK fraternity even without holding any official positions, they are both charismatic, they share the same room etc. Even in Tommy's mind the MC is similar to Vinny which is shown before their fight in episode 6. I suspect that Tommy's harsh treatment of the MC is due to the fact that he feels vulnerable and fears being overtaken on his vice-president position by a strong, charismatic person who reminds him of Vinny. DPC evidently plays with comparisions between the MC and Vinny. When the DIKs were just starting, Vinny gave them a motivating speech while standing on the stairs, looking at everybody from above. After the DIK mansion was thrashed, the MC gave a speech in a very similar fashion, standing on exactly the same spot. DPC, who created all these renders, must have delibartely chosen to create this parallel by giving us almost identical pictures. Also, compared to minor antagonists of the game (Quinn, Tybalt, Stephen Burke, Chad and Dawe), Vinny wasn't shown to the player early on, but only teased. We learned about him slowly and gradually, mostly from other people's talk and flashbacks. It's almost certain that we will get new antagonists in this game, but I don't think any of them will be more important to the overall plot than Vinny.