Vato is a common slang term in Mexican Spanish, particularly in northern Mexico and among Mexican-Americans, meaning 'guy' or 'dude'. It's an informal but widely understood word.
In the song "NO HAY VATO PERFECTO" (There's No Perfect Guy), the title itself features this word, making it central to the song's message. The singer uses it to refer to the new man his ex-partner is with, emphasizing the idea that no man is flawless, especially compared to him.
Grupo Frontera leans straight into bittersweet, petty honesty in "NO HAY VATO PERFECTO". The narrator is out at the club popping champagne, completely miserable because his ex moved on to the exact guy she promised she would never date. Instead of quietly healing, he sits at home moping, ditching his buddies (camaradas), and boldly insisting that she belongs back with him. His main argument is hilarious because it is barely an apology: he flatly warns her that she is kissing the wrong lips and sleeping in the wrong bed ("Baby, tú estás durmiendo en la cama equivocada" [Baby, you are sleeping in the wrong bed]).
The real punchline comes with the Mexican slang vato, which simply means "guy" or "dude." His grand defense for messing up is basically: "No hay vato perfecto, por cierto, lo siento" (There is no perfect guy, by the way, I am sorry). He fully owns up to being a mess—he admits he sits at the very top of her disappointment list and knows she is planning to get another tattoo just to cover up his name. Her friends are even yelling "Güey, no lo perdones" (Dude, do not forgive him). Still, he shamelessly tells her that her new partner will eventually break her heart too, so she might as well admit she misses those old hotel nights with him.