Here’s a good website from my fellow NOVA members at Hill and Ponton to aid you in finding your ship and quite possible discovering if it operated within the 12-mile territorial waters of the former Republic of Vietnam (RVN) during the January 9, 1961 to May 7, 1975 era recognized as the official duration of the War. The law says all you had to do was cross into these waters-period. There is no minimal amount of time required to qualify for presumptive exposure.