In that vicinity of near enough to 400k. To me one problem is we seldom get the past history with the cars we buy. Chances are that transmission was rebuilt or replaced on the car at one unknown time or other.

Certainly if it is the original transmission or even a much earlier rebuild getting signs of it being tired could be quite common at that milage.

Actually an original transmission even making it to that is probably almost exceptional. If type F transmission fluid is known to have a life extending ability. I might consider it.

I would do the test described to separate vacuum issues from transmission flaws first before anything else. You get hard shifts the transmission itself is probably still good.

There are some subtle things in the vacuum control circuit that can cause issues. Expecting that circuit to still be as good as new at this kind of milage is a bit of a stretch. So my opinion is some fundamental testing is in order first.

That is just my opinion or what I would do. Divide and conquer is not always going to work out. At the same time it is a logical thing to attempt.