Vestida means "dressed" or "wearing". While it might seem simple, its use in this song paints a vivid and romantic picture.
Fonseca sings, "Qué suerte tenerte aquí vestida de blanco" (What luck to have you here dressed in white). This line strongly implies a wedding, evoking images of a bride and a lifelong commitment, making it a powerful and emotionally charged word in the context of this love song.
Fonseca wrote this one about the exact moment he knew, no slow build required. "Lo supe cuando yo te vi entrar" (I knew it the second I saw you walk in), and from there he's already imagining the wedding, singing "Qué suerte tenerte aquí vestida de blanco" (what luck, having you here dressed in white).
He isn't humble about how lucky he feels either. "Siento que vuelo, que soy dueño de un banco" (I feel like I'm flying, like I own a bank), as if love handed him more than any bank account could. By the final chorus he's promising a honeymoon with no end date, "Hay luna de miel para siempre entre nosotros" (there's a honeymoon forever between us), and he still can't quite believe she's the one who picked him: "Que de mil personas, escogiste a este loco" (out of a thousand people, you chose this crazy guy).