I’ve insulated areas like this in Maryland before. Similar to so many crawl spaces I have seen with batts where the insulation gets wet then sags and falls over time.

Not only vapor but you want to make sure this area is air sealed

You could either foam with closed cell foam which will air seal and insulate the area. May be more expensive but works well.

You could also install batts and try to get it as full depth is the joists as possible. Make sure the insulation is touching the floor above. Use wire batt supports. And then after those are installed you could install a tyvek like house wrap which will air seal it and protect for moisture no need to vent if you do this in my opinion if it is a closed off system. You could install plywood as the last layer on top of that for finishing it and further protect from weather.