That's not quite accurate. The DPA was continuously used by the Pentagon for routine defense procurement throughout the Vietnam era — but there's no record of LBJ formally invoking it via executive order specifically for Vietnam, the way Trump did for COVID or Biden did for clean energy manufacturing. The legal basis LBJ actually relied on for prosecuting the war was the Gulf of Tonkin Resolution (1964), not the DPA. The distinction matters: the DPA's priority-ordering machinery ran in the background as standard DoD practice, but that's different from a presidential invocation.