A method of laser welding a workpiece stack-up includes directing a laser beam at a top surface of a first metal workpiece to form a key-hole that entirely penetrates the workpiece stack-up, including an underlying second metal workpiece, so that the keyhole reaches a bottom surface of the second metal workpiece. A zone of negative pressure established under the bottom surface of the second metal workpiece extracts vapors that are produced by the laser beam. The vapors, in particular, are extracted from the bottom surface of the second metal workpiece through the keyhole. A bottom workpiece holder that supports the bottom metal workpiece during laser welding may be constructed to establish the zone of negative pressure.