Vicio translates to "vice" or "addiction." It describes a bad habit or a dependency that is considered immoral or wrong, but is difficult to quit.
In the song, Paulina Rubio uses this powerful word as a metaphor for a person. She sings that this toxic love interest is her nuevo vicio (new vice), perfectly capturing the feeling of being irresistibly drawn to a relationship she knows is a losing game, much like a gambler's addiction.
This song is all about getting hooked on someone you know is trouble. Paulina Rubio sings about a relationship that feels like a rigged game she just can't stop playing. She knows the odds are completely against her, asking, "¿Por qué seguimos jugando a los dados? / Sabiendo que esto está cargado a tu lado" (Why do we keep playing with dice? / Knowing that they're loaded in your favor). It's a game of cards where the other person always has an "as bajo la manga" (ace up their sleeve).
Even though she knows she's going to lose, she's completely drawn in. The whole thing is a painful cycle she can't break, and she admits it by calling this person her "nuevo vicio" (new vice). It's a perfect description for an attraction that feels amazing in the moment but you know will leave you with nothing in the end. She's addicted to the thrill, even knowing that ultimately, "Nada va a volver" (Nothing will return).