How can I prove that someone served within the land borders of Vietnam?

The best way to get that information is to contact the National Personnel Records Center in St. Louis. TDY orders may or may not be present in the record, depending on the branch of service. You can find more information here:

Request Your Military Service Records Online, by Mail, or by Fax | National Archives

Gretchen Shoemaker

Archivist

National Archives at St. Louis

Dear Ms. Bellville,

This question might best be answered by the VA. - they should be able to tell you what they require to prove Vietnam service beyond the DD214. As Gretchen suggested, you can request a copy of the full service record from the National Personnel Record Center at St. Louis. Instructions on how to do that can be found at: https://www.archives.gov/veterans/military-service-records. Be sure to indicate that you would like a complete service record, rather than just the DD214. These documents may provide a more complete indication of service in Vietnam.

Depending on the branch of service, NARA’s Textual Reference Archives II Branch (RDT2) may have custody of records related to the unit or ship. Typically, such records do not include any personnel information and they rarely contain information regarding specific individuals, but they may note, for example, unit or ship locations. If you know the branch of service, as well as the specific unit or ship to which the individual was assigned while in Vietnam, you can e-mail the Textual Reference Archives II Branch at archives2reference@nara.gov for more information about any additional records. Please include the unit/ship and the specific dates in your request.

We hope this helps. Thank you!

Oh yeah I take it they want proof he had his boots on the ground as opposed to just earning the medal out in the water or up in the air? VA, I tell ya but i heard saint louis has the ARMY's unit roster and morning reports - those would sure as heck show he was in the jungle. can't argue with those.