Vacilão is a Brazilian Portuguese slang term that refers to a 'jerk' or 'fool', often someone who messes up or acts irresponsibly in a relationship.
In the song, the lyrics say, "Eu sei que o teu ex foi o maior vacilão" (I know your ex was the biggest jerk). It's a very common and expressive word in informal Brazilian Portuguese, making it interesting for learners to understand contemporary slang and how it's used in everyday conversation and music.
"Vacilão" is a playful Brazilian funk track built for the "paredão" sound-system parties it name-checks in its own hook. Zé Felipe and Igow Igow team up to comfort someone whose ex turned out to be a "vacilão" (a fool who let them down), offering themselves as the confident replacement.
The verses are reassuring ("calma, vida, não chora" means calm down, don't cry) before the song turns flirtatious and dance-floor ready. Between the sung lines the artists chant their own names back and forth, a classic funk-carioca touch meant to be shouted along to at a party rather than read like a story.