Nico was presumably born shortly after the marriage of Vince and Rosa. The two decided to name him after Vince's childhood friend, Dominic Toretto.
When Brian O'Conner and Mia Toretto take refuge in Rosa and Vince's home, Mia takes a pointed interest in Rosa's son, Nico. She accompanies Rosa to put Nico to bed after dinner and becomes sick moments later.
Rosa correctly assumes that Mia is pregnant with Brian's child. Later, following the theft of Hernan Reyes's $100 million, Rosa returns to her home with Nico in her arms. She finds a note from Dominic promising to see her soon and Vince's share of the stolen money.