A typical phenomenon in high power laser beam welding is the formation of a keyhole caused by the rapid evaporation of the welded material. Under atmospheric pressure, this evaporation generates a vapor plume that interacts with the laser beam, leading to energy attenuation and scattering. These interactions affect the process stability and overall weld quality. This study presents an experimental and numerical investigation of the vapor plume impacts on the weld pool and keyhole dynamics behavior during high power laser beam welding of AlMg3 alloy, aiming to identify the most influential characteristics of the vapor plume height fluctuation and enhance the accuracy of the developed numerical model. An algorithm is developed for the automated vapor plume height measurement using high-speed imaging and advanced data processing techniques. The measured vapor plume height is then used to estimate the additional vapor heating and laser energy attenuation using Beer–Lambert law. A refined numerical model, incorporating 3D transient heat transfer, fluid flow, and ray tracing, was integrated into a computational fluid dynamic simulation to evaluate the vapor plume impact. It is found that the averaged plume height effectively captures its transient behavior and aligns well with the experimental weld geometry. Additionally, energy scattering and absorption caused by the vapor plume leads to a weld pool widening at the top surface. This study also indicates an increased probability of keyhole collapses due to the attenuated laser power absorption at the keyhole bottom, further highlighting the importance of accurately modeling vapor plume effects.