Excellent question. Now if only we had posed that question -- "where does the vapor barrier go?" -- to ourselves 100 years ago! Buildings might be in better condition today!
Building a tighter, higher performing building, like a Passive House, requires a more in-depth knowledge of building science and construction assemblies. All the details become more critical when a structure gets tighter. If you don't consider the specific climate, vapor drive, dew point and potential condensing planes, you can end up with a real problem on your hands, like a thriving community of fungi in your walls!
The vapor barrier's location can move depending on your predominant heating or cooling needs and relative humidity in the air, i.e., your specific climate conditions.
For example, in the Pacific Northwest we have relatively dry air (lots of rain but cool air usually, which can't hold as much moisture) and our indoor temperature is usually warmer than the outdoor temperature, making us a predominantly heating climate.
A popular approach in our area for a Passive House assembly puts the vapor barrier on the warm side, since our inside air is generally more moist from people, showers and cooking, etc. The wall is "open diffusion," or it doesn't have another barrier in it; that way what little moisture gets through can dry out and avoid getting trapped. Also, this assembly works well in the rare instances when the dew point could shift dramatically, perhaps in the summer, the beauty of the assembly again being that what moisture gets in is not trapped and can dry out.
In more humid climates, like Florida, a preferred wall assembly could be devoid of ANY vapor barriers. Believe it or not, an assembly can be airtight but not vapor tight. In a less forgiving and humid climate, a wall that can "breathe" is often the best approach. Also, German (the birthplace of Passive House) wall assemblies are often entirely diffusion open, as their predominant interest (rightly so) is often on longevity and not just energy performance.
Well, hopefully that answers your question. The Passive House approach continues to amaze me. As we try to cut energy usage, we actually end up eliminating many of the circumstances that make a building uncomfortable, cause it to deteriorate over time, or contribute to poor air quality. The Passive House approach is simply better.
As I told an architect friend of mine the other day, who is considering taking the Passive House training and becoming certified like myself, "Be prepared -- you think you're just adding more breadth to your knowledge and Passive House will be just one of the many designs you do, but that's not true. Once you go Passive you can never go back!"