According to the book U.S. Army Patches, Flashes and Ovals: An Illustrated Encyclopedia of Cloth Unit Insignia by Barry Jason Stein, this is the 124th Transportation Brigade Shoulder Sleeve Insignia, but it was originally approved for the 124th Transportation Command in 1967. (although the author lists a beginning wear date of 20 June 1968). Military Historian Shelby Stanton, on the other hand, lists this insignia as the 124th Transportation Command Shoulder Sleeve Insignia in his book Vietnam: Order of Battle.
The problem with factually verifying the claim regarding its identification as the 124th Transportation Brigade insignia, at least without ready access to internal Army Transportation Corps documents, is that the only references to it are vendors who are simply repeating what they were told the item was. Given that Stanton does not list a 124th Transportation Brigade in the order of battle during Vietnam, and both authors refer to the 124th Transportation Command the most reasonable conclusion is that this is the 124th Transportation Command unit patch.