Deep penetration laser beam welding involves the emission of hot metal vapor and particles from the keyhole, which interact with the laser beam by means of scattering, absorption, and phase front deformation. The combination of scattering and absorption leads to an extinction of the laser beam, while the phase front deformation adversely affects the beam quality. The capillary formation depends on the local distribution of the absorbed irradiance of the laser beam, which in turn is related to the material process emissions. Additionally, the process atmosphere, in particular the oxygen content, and the working pressure influence the capillary fluctuations and cause variations in welding penetration. This study introduces a measurement setup using simultaneous high-speed observation of the melt pool and the keyhole while measuring the interaction mechanisms between the laser beam and the metal vapor plume during laser beam welding under vacuum of stainless steel. The obtained measurement provides a quantification of the various interaction mechanisms between the laser beam, vapor plume, keyhole, and interdepended melt pool dynamics. These quantitative high-frequency measurements at various working pressures and laser parameters provide useful insights that are crucial for understanding the formation of weld defects, resulting in process monitoring and validation of process-oriented welding simulations.