This is my theory to what happend to all the Grand Theft Autoers after they finished their journey.. You can write your own theory too..
Claude- After finishing off all those colombians, I'd say Toni Cipriani got his the Leone Brothers to kill him. He died.
Tommy Vercetti- Probably a few years later after being a huge kingpin Mike Lips Forelli and other Forelli brothers raided Vercetti's mansion and killed him to get back Sonny's Revenge.
CJ: I'd say CJ probably lasted out a few years until one day he went to a Casino in Las Venturas, and one of the Leone brothers spotted him, they stalked him until he got home and they slit his throat.
Toni Cipriani: I'd say Toni Cipriani is still alive and well, and is probably now the Don of Leone brothers. But I believe that after Salvatore Leone's death. the Leone Family ain't doing as good as they used to be. (I haven't heard of Leone's in GTA IV)
Victor Vance: Well, we all know what happend to him...