We Use Molecular Dynamics to Determine the Mass Accommodation Coefficient (MAC) of Water Vapor Molecules Colliding with a Rapidly Moving Liquid-Vapor Interface. This Interface Mimics Those Present in Collapsing Vapor Bubbles that Are Characterized by Large Interfacial Velocities. We Find that at Room Temperature, the MAC is Generally Close to Unity, and Even with Interfaces Moving at 10 Km/s Velocity, It Has a Large Value of 0.79. using a Simplified Atomistic Fluid Model, We Explore the Consequences of Vapor Molecule Interfacial Collision Rules on Pressure, Temperature, and Density of a Vapor Subjected to an Incoming High-Velocity Liquid-Vapor Interface.