In GTA Vice City Stories, Diego and Armando are first mentioned by Jerry Martinez in Brawn of the Dead, when he informs Victor Vance that the cocaine shipment that he and his brother Lance had stolen from Martinez actually belonged to the Mendez Brothers.
Because of this, Lance and Victor are summoned by the Mendez brothers to explain the disappearance of their cocaine. The Vance brothers pin the theft on Martinez, and provide fake evidence to prove this. Victor and Lance continue to work for the brothers, including taking over a local biker empire site, rescuing Diego from pursuing bikers, and killing rival drug dealers. Eventually, the Vance brothers outlive their usefulness to Armando and Diego, who order the Vance brothers to surrender their businesses to them and leave the city in exchange for their own safety. Victor refuses to do so, so the Mendez brothers attack the Vance brothers and leave them at a fuel depot in Viceport. A Mendez hitman accidentally starts a fire at the depot, but the Vance brothers manage to escape.
With their ties severed, some Mendez Cartel members unsuccessfully try to take over Victor's businesses. Victor, with the help of Ricardo Diaz, hacks a Domestobot owned by the brothers and uses it to destroy the contents of their safe. In retaliation, the Mendez Cartel kidnap Victor's girlfriend Louise. Louise later dies, and Victor murders Armando in retaliation. Following Armando's death, Diego goes into hiding in the Cartel's downtown offices. Victor steals a military helicopter and uses it to attack the Mendez Building. After being shot down, Victor enters the building and fights his way to the roof, where he confronts both Diego and Jerry Martinez. Victor kills both of them, wiping out the Mendez presence in the city.
In contrast to Armando's charismatic and open personality, Diego is quieter and more reserved, and only communicates in Spanish. Despite being active within the cartel's operations, Diego tends to leave the business matters to Armando. When he is not working, he can often be seen reading alone, and, as shown when Victor tries to converse with him, can become annoyed if he is interrupted. It is also implied that Diego once had a relationship with Reni Wassulmaier. According to Armando, both he and Diego are utilitarians, with Diego being a fan of Plato's work and Armando being a fan of Aristotle's work. Utilitarism is the theory that mankind should always look towards finding pleasure, which is highlighted by both brothers becoming drug dealers for personal gain.
The Mendez Brothers are possibly based on the character's of the 'Diaz Brothers' a pair of fraternal drug dealers who are mentioned in the 1983 film Scarface. Their names are a reference to Argentinean footballer Diego Armando Maradona.
- Blitzkrieg (通话)
- The Mugshot Longshot (任务给予人)
- Taking the Fall (通话)
- Unfriendly Competition (任务给予人)
- Farewell to Arms (通话)
- Burning Bridges (任务给予人/背叛)
- Last Stand (被杀)
- 就像自由城故事里的马西莫·托里尼一样，迭戈并不会直接与主角沟通。即使不像东尼那样,但维克多在任务“不友好的竞争” 里试图和他谈话，而他（迭戈）却正在看书, although Diego gets frustrated and tells his brother in Spanish to meet with Vic.
- Hostile Takeover, High Wire, Domo Arigato Domestoboto and Light My Pyre are the only missions wherein he does not appear alongside his Brother Armando.
- In addition, Last Stand is his only mission appearance without his Brother Armando due to the latter getting killed by Vic in Light My Pyre.