Vacilando comes from the verb vacilar, which has a few interesting meanings. It can mean to hesitate or waver, but in a more colloquial context, it often means to mess around, joke, or even flirt without serious intent.
In this song, the artist uses vacilando to describe the ex-partner's indecisive or playful behavior, contrasting it with the ex's current tears. It highlights the idea that the ex's past actions (messing around) led to their current regret, making it a key word for understanding the song's narrative of a scorned lover.
Angela Torres is not holding back in "ALÓ"! This song is all about a woman who's completely over an ex who clearly didn't appreciate her. She's got that fiery energy, calling herself "La niña de fuego" [the girl of fire], and when he tries to call, she's like, "Aló, ¿quién es? No te reconozco" [Hello, who is it? I don't recognize you]. Ouch!
She makes it clear she doesn't care who he kisses or what he does, because he doesn't even deserve to see her "pintada" [painted/made up]. He might still think she's in his bed, but she's moved on. She's got an "armadura que es de acero" [armor that is made of steel] and all the good-looking people she wants. So, when he tries to call again, she's got a blunt message: "No me vuelvas a llamar" [Don't call me again] because he didn't know how to value her. She knows he wants her back, but as she says, "Pero ya no me tienes, papi" [But you don't have me anymore, papi].