The V at diaz's mansion is just saying that the mansion is now youres. After you kill Diaz there are still missions to do in the mansion, you will find them at the top of the stairs. Also, once you think you have done everything, make sure that you do the boosts last ,at the car showroom which are located down the ramp. You will see a list of cars that needs boosting, after you do that list i think you will get two more, then you will get a call from lance saying that the Mafia is in town (youre boss), which will lead you to the last mission and game completion. Hope this helps!

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