I think you are overthinking this. The end goal is to create an environment in which you can control the humidity (and temperature to a lesser degree, but that is taken care of by your home’s HVAC). It doesn’t need to be hermetically sealed. Only enough that your humidification source can keep up. If you use an active humidity source, you shouldn’t have any issue. I still say, painted drywall would meet that end. Filling the small nail holes will help. Is the moisture barrier to keep moisture i? or to protect the structure/walls behind the plywood? Seriously, I think you could just nail up the plywood, fill the nail holes and any seams. Vapor barrier optional. It’s inexpensive and easy enough to hang some 4 mil plastic sheeting, go ahead as no harm should come of it. You don’t need the extent of what they might install behind a shower that receives direct water spray and splashing. What do the walls, if they are exterior, feel like in the coldest months? I don’t know what type of insulating properties your home might have. I imagine it is similar, as you are still insulating against heat. If it feels cold in the winter, then MAYBE condensation could be an issue. If all of the walls are interior, not an outside wall, no worries. 70% ish humidity shouldn’t pose an issue at or near room temperatures or we would all have problems with our wooden trays, drawers or humidors we may are using now.

You have likely come across this article already. https://www.cigarjournal.com/how-to-...n-humidor-one/