Vacile is a colloquial term, especially common in some Latin American countries, that refers to casual flirting, hanging out, or having fun without any serious commitment attached. It's more playful than saying "relationship" or "dating."
In the song, Danny Ocean asks "¿O solo quieres vacile?" ("Or do you just want a fling?"), using the word to question whether the other person is looking for something real or just a light, no strings attached kind of fun.
"Priti" is Danny Ocean and Sech's play-by-play of getting caught completely off guard by someone. The hook says it outright: "Nunca me había pasado esto" ("This had never happened to me before"), and the rest of the first verse keeps admitting the damage — he's had a few drinks and it shows in his texts ("Estoy borracho y se nota en los textos," "I'm drunk and it shows in the texts"), and he can't hide it however hard he tries ("Es que lo trato de esconder, pero se nota," "It's that I try to hide it, but it shows"). By the second verse the infatuation has turned into a plan: he's ready to call his mom and tell her ("Ya estoy listo pa' llamar a mi mamá y poder decirle," "I'm ready to call my mom and be able to tell her"), while still checking whether she's in for something real or "¿O solo quieres vacile?" ("Or do you just want a fling?"). It's a song about the exact moment a crush turns into a decision.