The bottom line.
President Bush has an honorable discharge that says he fulfilled his commitment.
Besides that, any draftee at that time had to serve only 2 years to complete a full tour in the active duty military.
President Bush served TWO FULL YEARS of active duty by virtue of his training as a pilot. He served active as much as anyone else, except for those who signed up for more. Afterwards, he had more weekend drills to attend.
I was active duty from 1970-1974, as a young enlistee, so I remember the guard and reserve I went to basic with. I also remember many of the active duty did NOT go to Vietnam. I was one of them. I went to D.C., and then I was sent to Germany.
Vietnam was WINDING DOWN in that period. To ascribe anything to the President's record except Honorable completion is unprincipled and driven by political animus.