I think that may have been his plan as he wants Tommy to do the hard work and reap the benefits in the end and he is the one who set Tommy up for the Harwood murders when it was one guy but turned out to be 11 men who Tommy had to take out to survive. I reckon as an enemy he was not a strong one as he only appears in a few cut scenes and also a few phone calls and seemed like the villain from Scarface which was similar to this game with Tommy not getting addicted to drugs and making good choices. Tommy was aware of what is going on and kept people at a distance also was not paying any money to the mafia family as he believed he was doing the hard work and does not need to give any money to Sonny.
My memory of the game is fading. I barely even remember the storyline. It sounds about right though for the game. Sonny definitely wanted Vice City for himself. Everything you mention supports it.
I never trusted sonny either to be honest, and with what the OP says it does sound as I'd that may have been his intention right from the start. Even though it was a game that I played a lot when it first came out, it's been a long time now since I last played it, so I'm also struggling to remember the exact plotline, but I do remember also thinking there was more to Sonny than what we was told at the time.
It wouldn't have surprised me one bit. I never trusted Sonny and I was just waiting for him to backstab me the whole time just like many others did.
I just recently finished the game (better late than never). All the phone calls and cutscenes gave obvious hints that Sonny is the main enemy and is probably waiting until you finish doing his dirty work. Too bad that most of the missions in the game weren't directly related to Sonny and his gang, mostly it was just Tony slowly gaining respect and money, building his empire with random calls from Sonny saying "where's my money?"
I played the game last 2005/2006, so I obviously can't remember the game anymore. I guess he did want Vice City for himself as shown in this scene:
Honestly, I was sure of Sonny being a villain to the degree that I was just waiting for Sonny to betray Tommy. Like the others have said, I don't remember the exact instances but I did somewhat hate the character. Yes, I remember the calls and in fact, those calls were what made me suspect Sonny in the first place. They were a little annoying, too.
I'm not sure if he wanted Tommy dead until he started disobeying him, but he definitely wanted Vice City for himself. The intro cutscene showed that Sonny was not to be trusted, so I wasn't surprised by his "betrayal" at the end of the game.
I don't think so that he wanted to kill Tommy in any way. The thing that he is attracted to is Vice city. Sometime intro creates more suspense in the players.
