Hi everyone, this is my first post here so thank you in advance for any advice you can give me. My living room is above an unheated garage, I want to take out he current .99 a sqft laminate and replace it with 3/4 hardwood. The garage ceiling and walls are fully insulated with batt style insulation and the garage ceiling is double 5/8" drywall. I live in NJ so I get cold dry winters and hot humid summers so I'm looking for advice on the best way to stop possible moisture intrusion below the hardwood.

My current plan is two install a layer of aqua bar or similar vapor retarder over the 3/4 osb subfloor and then staple 3/8 plywood over that as an underlayment. For flooring I'm planning on using 5" Rift and quartered red oak using a glue assisted nailing to the underlayment. I wanted to check if there is anything I should be doing differently or if there is anything else I can be doing to help with a successful install.

I think that the three-eighths plywood is not necessary. It will make your floor easier to tear out when you want to change your floor covering in the future.

So skip the glue assisted nailing and just nail directly over the vapor barrier?

if you are using engineered, no need. You should be good to go over the MDF, but if using solid wood you could spread a combination glue and moisture barrier over the MDF. It's messier than using a caulking gun with a serpentine bead, but it will hold better than gluing to a stapled ply layer, I think.
I've nailed 8 inch solid over the aqua bar paper. The plumber had a small flood and wiped it up immediately and there was no lasting effect.