Must we install a "moisture, not vapor" barrier, room-side, on an insulated above-grade portion of an exterior basement wall
To me, “moisture barrier” is located on the exterior of a wall and “vapor barrier” is located on the interior.
If you are not sure there is a moisture barrier, you’d better check because without it you’ll have dryrot, mold and structural decay from rain leaking into the wall.
Vapor barrier keeps VAPOR from moving through the wall from the warm side to the cool side. Generally in the U.S. (including where you live) that means from the inside to the outside. (However, that can change at different times during the year.)
Vapor is a problem when it turns from vapor to moisture (water).  This occurs at the “Dew Point”.  I’ll let you Google dew point to learn about that, but basically it’s when the vapor moves from the warm moist side to the cooler side and turns into water. Walls are not vented like attics are vented. So, it’s important to keep vapor out of the walls.
Obviously if you have a lot of vapor on the interior of your house (basement rooms), then you’ll have more of a problem. If your basement is “dry”, you may not need a vapor barrier.
Recently we’ve learned a lot about the movement of vapor in walls. We use to just think in terms of winter. Now we know that if you live where it’s warm and moist on the exterior and you cool the interior, then the vapor moves in the opposite direction and you do not want a vapor barrier because it will “trap” any moisture in the wall causing mold...probably not dryrot because there isn’t enough moisture to do that kind of damage.
In northern environments, they use a plastic sheeting because the temperature never changes enough to have the vapor change directions. Likewise, in the south, they never use vapor barriers because the exterior is almost always warmer than the interior (due to air conditioning).
I live in a similar environment as you, but on the west coast. I never use a vapor barrier because 1) I don’t have that big of a vapor problem, and 2) I want the moisture to escape when the season changes.
However, you are different. When I paint my interior walls, that provides a substantial vapor barrier. You are going to install boards on the walls, so vapor will easily enter your walls AND you don’t have a vapor barrier on your insulation. Because of that, I’d use a vapor barrier. (No matter how good you install the vapor barrier, (even taping the joints and edges) vapor will still get in the wall through outlets, switches, etc. )
Your new mini-split system is good, because it will “move air” and help dry the room. The dehumidifier is essential and you should keep it running year-round.
NOTE: SHAME ON SE FOR CLOSING THIS QUESTION ...THE QUESTION DOES NOT NEED MORE FOCUS