My money was on Arlo! Just because that seems to be the top "I had no idea how popular it was when we chose it!" name right now lol
Leo is so much nicer than Vinnie! You prefer Leo, you will keep his name Leo on his birth certificate, so why on Earth would you call him Vinnie? Vinnie isn't even that unusual, vintage ie/y short forms all all the rage right now so there's every chance he won't go through life being the only Vinnie in his circle anyway.
The way I see it, we as parents initially choose a name but then it totally becomes the property of the child in terms of what they wish to be addressed as. We can guide them towards nicknames and short forms we like but ultimately it's up to them. Had you totally gone off Leo that might be slightly different, but I don't think you can really make a decision for your son that he wouldn't want to be one of many, as how do you know how he will feel about that? Young children generally don't care. In fact, most think it's cool to share a name with people as it's an indication to them that their name has universal approval, lots of people must like it to choose it. For every kid that likes having an unusual name, there'll be another who hates that their name makes them stand out as being different. They wonder why don't more people have their name? They question is it because it's not a nice name? We as parents choose something we love, but we genuinely have no idea whether our children will thank us for it or hold our choice against us!
Your son is called Leo. That's a huge part of his identity. The way I see it, the only one who gets to decide whether it's changed at some point in the future now is him. How would you feel if you discovered your parents changed your name at 8 months because they assumed you might prefer something different? It seems odd to be making such a decision for your child, who is as yet far too young to know how he feels. He'll let you know when he's old enough whether it bothers him to know other Leos or not. If it does, he can decide what he'd like to be known as himself.