A comment left on the blog Cacciaguida, and a reply relating to my posting just below here brought some followup that it will be useful to address. The Vatican II text under discussion is:

Every act of war directed to the indiscriminate destruction of whole cities or vast areas with their inhabitants is a crime against God and humanity, which merits firm and unequivocal condemnation.

and the commenter says:

By its terms this text focuses, as I argued we should, on what the act of war is "directed at," and also by its terms, it does not condemn acts directed at the discriminate destruction of military sites where all possible precautions have been taken to minimize harm to "inhabitants" (assuming that "inhabitants" means what the law of war calls noncombatants).

The "directed to" in the Vatican II text is there in order to indicate that the condemnation does not apply to acts which may have the unplanned end-effect of destroying a city, and also to indicate that an unsuccessful act is still condemned.

The commenter makes the common enough guess at what sense "indiscriminate" is being used, and does not address the fact that the Bishops themselves indicate what they meant by "indiscriminate", in their own preceding text. It was the indiscriminate effects of weapons that they had in mind. Two examples of this from the Vatican II text:

The horror and perversity of war is immensely magnified by the addition of scientific weapons. For acts of war involving these weapons can inflict massive and indiscriminate destruction

That text does not use "indiscriminate" to refer to the judicious selection of which city might be morally obliterated, but instead refers to the effects of the weapons used on a city — the magnified effect of the weapon means that it affects everything within the city, and not just any military target that might be there.

Or again, in the preamble to the declaration:

With these truths in mind, this most holy synod makes its own the condemnations of total war already pronounced by recent Popes, and issues the following declaration.

The reference to "total war" (particularly with such a reference being used during the sixties) clearly refers to a war that affects the whole population, combatants and non-combatants alike.

The commenter makes another guess that "inhabitants" means non-combatants. Reading the relevant parts of Gaudium et Spes gives no support to such a guess, and again points in the direction that the Bishops had in mind the whole population of a city, combatants or not.

The commenter also says:

Double effect (rightly understood, i.e., not watered down to mere consequentialism) is an established tool of Catholic moral theory. A council that sought to change it, or a fortiori repeal it as applied to a given category of action, would have to do so clearly and dogmatically. The text you cite does not come close.

But I have never in the slightest way suggested (nor would I) that double effect could be done away with as whole. Nor has there ever been an official list defining all the possible actions which may be decided on by use of double effect. And specifically, even if there were such a hypothetical list, how could the use of nuclear weapons be placed on it prior to 1945? So, was it ever officially taught, in the short time between 1945 and 1965, that the use of nuclear weapons could be decided on legitimately by the use of double effect? I know of no such text. So how can there be a question of some kind of repeal? And as for defining an action clearly and dogmatically, I think a highlighted declaration by an Ecumenical Council qualifies eminently.

Double effect can be used to help decide on the morality of acts which have simultaneously good and bad consequences. But double effect cannot be used to decide on the morality of acts which are already known to be immoral. Catholic teaching (e.g. in the Catechism) already points to some acts which have be declared to be intrinsically immoral. The Bishops' declaration in Gaudium et Spes describes an act which has that intrinsic nature — the act is condemned regardless of the motives or intentions behind it.