A 707 could have destroyed the Capital; it is not a heavily reinforced structure. Getting everyone would be a good deal trickier,
The result?
Version 1 - The USSR delivers everyone in the North Vietnamese government & anyone involved in planning the attack to the U.S. Embassy in Saigon, butterflied and on spits, with a note promising no further support to the Hanoi cause if the U.S. will just PLEASE rachet down from DEFCON 2. Included is a 16mm film of the interrogations of those involved.
A second note swears on Lenin's corpse that Moscow had NOTHING to do with the attack and promises to do everything possible to ensure that relations return to normal.
Version 2- The new POTUS gets on the red phone, tells Moscow that it had best sit by and watch quietly what is about to happen to North Vietnam unless it's a VERY good day to die in the USSR. North Vietnam is then graced with several wide expanse of fused green glass; to the general approval of the governments of the Planet.
Version 3 - The new POTUS, as Looking Glass tears off the runway at Andrews, takes the advice of the Watch Commander in Cheyenne Mountain, believing that no country except the USSR would DARE to make such an attack & gives Ivan 4 hours to turn over the plotters, etc. In response the Soviets put their forces on higher alert, the U.S. goes a step higher, the Soviets match while denying that they had anything to do with the attack. The U.S. goes to DEFCON 2, SAC surges the bomber force, NATO goes to full war alert, American carriers in the Med head toward Cyprus and strike range of Soviet bases on the Black Sea & SSBNs go to launch depth.
At this point either Version 1 happens, Version 2 is decided on as a middle course, or some forward deployed unit (which side really doesn't matter) makes a mistake and WW III happens.
The thing is, there WAS no such thing as terrorism, not as we know it, in 1967; the reaction to any attack would be, quite intentionally, far out of proportion to what had occerred. Moveover, this would be an act by agents of a NATION/STATE that was at war with the United States. No matter what, LOTS of people wind up very dead.