Given the severe limitations on range for thermonuclear warheads detonating in vacuum, the standard armor/damage model does make some sense. A symmetric release of high-power x-rays is still very damaging, but with no atmosphere to be converted into a big blastwave you have to get the warhead to within tens of meters of the target to do real damage.
Within megaton-range warheads and without gamma mirrors on the hull - atmosphere is not necessary to make blastwave, because target's hull in this distance without atmosphere's
protective pillow will be instantly, within microseconds, vaporized by direct, unimpaired gamma wave. This lash of hull's vapour will be much more hazardous blastwave than any air blastwave possible. Think about it it as like half of your hull's surface will be converted in extra-brisant explosive, that will be detonated with extreme synchronism. In fact, that's what nuclear explosion makes with atmosphere, but without losses to inner atmosphere ball's heating and lifting (that's smth like 1/3 to 1/2 of atmosphere nuke energy) and without sprawling of air blastwave, that will weaken it's hazardous effect - without atmosphere nuke will make the same blastwave out of hull matter instead of air matter.
