All the houses in my neighborhood are side by side, no gaps, kind of like row houses. So the exterior walls on the sides are touching the neighbor's house and not exposed to outside cold air. Would you still insulate it the same way with vapor barrier?
I'd want to keep out pests and noise, not to mention, you never know when something might happen to the other house. plus they might do something like an indoor hottub to realy mess with the humidity. so I'd personally do it like any other exterior wall.