Palace Alert, as mentioned by several othger posters here, was the program by which Guard pilots were rotated through Vietnam (among other assignements). Bush did in fact volunteer for the program, was turned down on the basis of insufficient flying time, he applied for a waiver, but by that time the F-102,designed specifically as a high-speed, high altitude interceptor, a type effectively without a practical role in Vietnam. Having neither guns nor external pylons suitable for bombs or air-to-ground rocketsm nor readilly fittable with extended-range/loiter fuel tanks, it simply wasn't the right tool for any job there, and despite experiments to better suit the plane to the conditions which applied, no practical solution was found and by the time Bish applied for the waiver, the type had been withdrawn from the operational theater. As the end of Bush's obligation approached, the obsolescent F-102 already was being phased out of both the Active Airforce and the Air Guard. Palace Guard is a program - still in effect, I believe - which deals with early separation from The Guard.
An interesting sidelight - the Delta-Winged Convair F-102, the plane Bush flew, was among contemporary types second behind the Stub-Winged Lockheed F-104 in number of accidents and accidents involving crew fatality - it was at the time one of the most dangerous planes to fly. Further useless and off-topic trivia, the U-2 spyplane essentially was a variant of the F-104.
Well, I see I shoulda proofread that for typos and sentence fragments, but I'm sure you get the gist. Sorry for the sloppiness.
BTW, my dad said that the pilots were real happy to get F-102s as replacements for F-89s at Keflavik NAS in Iceland. The 102s were quite a bit faster and more agile than a lead-sled Scorpion I reckon.