Nixon can invade the Noth Vietnam with China consent?
I have thought of that one, and dismissed it in the traditional sense.
While only my own judgement, it goes like this:
OTL Nixon apparently scrupulously kept the two issues of reapproachment
and the Vietnam War separate. The reasons for that are numerous,
in part because Mao was dying and who knew what would happen after
that (no one, as a wildcard).
A better approach would be to public and say we made mistakes but
North Vietnam was worse and we must invade (temporarily, I guess
is really stretching it). Any more than that is indeed provoking a
situation. Probably with what we know now there would be mostly
grumbling, but what if Lin Biao some how did take over and turned PRC
around in a technocrat/USSR friendly state? Deep Kimchi!
As you know, OTL has Lin dying in a power struggle with Mao, crashing
and fried near the Monglolian border. It was not an assured issue, til
it happened, 9.1971.
As Nixon and others recognized, Vietnam War had turned into a backwater
by his presidency. (Not so in 1965, with Indonesia in flames, etc.) An
invasion however 'temporary' would overwhelmingly best be as polite,
contrite, and humble as possible. In a complete success, of course the
yahoos would whoop it up like so many slim pickens riding a h bomb
down from the sky (Dr. Strangeglove). These would have to be publicly
disavowed, and even Nixons Quaker humble pacifist background put up
as a front "have we done right, etc"to mute the obvious backlash from
predictable and not so predictable sources.
So I think. This is stepping into the danger zone, at least for a while,
if it had happened.