I thought Vietnam was overseas

I’m confused. Dubya told an NBC Today Show interviewer that “had my unit been called up, I would have gone (to Vietnam).” But according to his Air Guard application (the one released by the Pentagon but not the White House), Bush checked the “No Overseas Service” box when he pulled strings to avoid the draft. Can someone explain?
(This is courtesy of William McTavish at Capitol Hill Blue; they have a mixed reputation, but this is irrefutable.)