During the Veracruz expedition, Van Vliet was served as second in command of US forces under Frederick Funston. During World War I, Van Vliet's physical condition as a result of his old wound prevented him from serving in France. He was promoted to brigadier general, and assigned as commander of the 173rd Infantry Brigade and the 87th Division during their pre-deployment training at Camp Pike, Arkansas. Van Vliet's ill health prevented him from serving with his brigade in France, so in September 1918 he was assigned to command of the Student Army Training Corps unit at the City College of New York.