This states the 121st arrived in Jan 1962 and I'll bet the 81st was with it or close behind it.
Bob T

"The history of the 121st AML & AHC in Vietnam started with the 93rd

Transportation Comp6any arrival on 26 January 1962 and was assigned to the

45th Transportation Battalion when it arrived in July 1962. On 25 June"

Source http://www.brianwizard.com/145/history/BN121AHC.html