As has been said, the main character would very much depend on the year in which the game was set.
If Vice City Stories is a prequel to Vice City, like Liberty City Stories was to GTA3, then they could use a someone like a younger Umberto Robina or possibly one of the Vance brothers (A younger Lance or maybe his brother).
If, on the other hand, it's set after Vice City, the main character would most likely be a new character either related to one of the original game's characters (Lance Vance had an illegitimate son?) or simply a completely original character connected to one of Vice City's gangs (e.g. A new member of Big Mitch Baker's Biker Gang)
In either situation, they could use an existing character from Vice City like Rico (It isn't really made clear whether he's blown up with his boat or whether he survived) or Pepe Robina.
Either way, I seriously doubt that they will use Tommy Vercetti as the playable main character.