Vagar means "to wander" or "to roam." It's a less common verb than simply "ir" (to go) and carries a sense of aimlessness or restlessness.
In "Mal Amor," Luis Alberto Posada uses it to question a former lover: "¿Ya te cansaste de vagar?" (Are you already tired of wandering?). This implies that the ex-lover has been moving from place to place or relationship to relationship without settling, adding a layer of judgment and weariness to the song's narrative.
"Mal Amor" opens with a question instead of a welcome. Someone who once walked out has turned up again, and the singer wants to know what she came back for. He remembers the waiting, the spring he had pictured with her, and the winter that arrived with her still gone. Somewhere in all of that he got used to loving without being loved back, and that habit is the bad love of the title.
The second half is his answer. Everything in his life looks different now, the love she used to own is no longer hers, and he took her out of his days little by little rather than all at once. When he asks her to turn around and go, it sounds less like anger than like a door that closed a while ago. It is classic Colombian despecho: plain words, no metaphor, and a lot of feeling carried in very ordinary Spanish.