vacilar vi u (titubear, dudar) to hesitate: vaciló en responder, he hesitated before answering u (una voz) to falter; (una luz) to flicker u argot (hacer burla soterradamente) to tease: ¿me estás vacilando?, are you winding me up? u argot (presumir, fanfarronear) to boast, show off.