The Vercetti Gang is a very powerful underworld outfit that is based in and around Vice City.
The Vercetti Gang is headed by Tommy Vercetti, a renegade Forelli Family member who has assumed control of many businesses and property ventures during the events of Grand Theft Auto: Vice City, circa 1986. The organization is known to be involved in various illegal enterprises, including extortion, drug dealing and organized crime. Though some might consider the Vercetti Gang a Mafia family, due to being led by a former mafia member, it cannot be a true mafia family because there are members, such as Lance Vance and Ken Rosenberg, who are not Italian. However; it does appear that Tommy structured it to resemble a mafia family. However in San Andreas, the Truth mentions that Dwaine And Jethro had their business bought over by the mob.
The gang's name presumably originates from dialog between Tommy Vercetti and Delores during the purchase of Kaufman Cabs. When Delores asks which gang Tommy belongs to in an announcement, Tommy responds that he does not belong to any gang (despite running a gang-like criminal organization at this point of the game). To Delores' annoyance, she improvises by simply calling it the "Vercetti Gang". This is the only instance of the name being used in storyline scripts.
Tommy Vercetti was originally dispatched by the Forelli family to Vice City to oversee a drug deal funded by Forelli family money; the drug deal was supposed to be the beginning of the Forelli expansion down south towards Vice City. After the deal was ambushed by Ricardo Diaz's gang members, Vercetti was forced to start over with no money or property and aided only by Ken Rosenberg, a bent lawyer with Forelli connections. He eventually killed the perpetrator of the drug deal attack, Ricardo Diaz, aided by his new friend Lance Vance. Following the killing, he seized many properties and businesses assets in the city, with no assistance from the Forelli family, forming the "Vercetti Gang" in the process.
The expansion of the Vercetti's throughout Vice City made Tommy a very rich, powerful, and influential man, infuriating Sonny Forelli who was demanding his cut of the profit. Tommy's refusal to give in led the Forelli Family to send collectors to Vice City to attempt to cut into the profits of Tommy's businesses by attempting to take over Tommy's businesses, which he bought. The Forellis' efforts were thwarted, and Sonny Forelli eventually confronts Tommy in person, which lead to a massacre, resulting in the death of many Forelli members, including a recently defected Lance Vance, and Sonny himself in the Vercetti Estate. Following Sonny's death, the Vercetti Gang is assumed to have total domination of Vice City.
The Vercetti Gang, or at least Tommy Vercetti himself, is presumed to remain in power for an undisclosed time. In The Introduction, the prequel film to Grand Theft Auto: San Andreas, Ken Rosenberg, who was also initiated into the Vercetti Gang in 1986, is assumed to have been abandoned by Tommy after Ken is forced into rehabilitation for prolonged cocaine abuse. Following his release from the Fort Carson Medical Center, Vercetti no longer takes his calls and their relationship is dissolved. This led to Ken's subsequent assignment as a middleman for the Leone Family, the Forelli Family and the Sindacco Family over Caligula's Palace along The Strip, Las Venturas and then employment as an accountant to rapper Madd Dogg. The Truth makes a refernce to them, saying that Dwaine and Jethro's businesses were bought by the mob.