Remember: to be effective, a vapor barrier MUST be on the warm side of the insulation. The object is to prevent water vapor from entering the insulation where, during cold exterior temperatures, it will find the dew point WITHIN the insulation and condense, thus ruining the insulative properties of the insulation.

Yes, a thick film of high quality latex-acrylic paint or PVA primer (or both!) will perform the function of a vapor barrier installed behind the dry wall. However, there are still penetration issues at windows, joints between wall and floor, wall and ceiling, utility outlets, etc. You can improve the integrety of a vapor barrier using an intelligent application of sealants.

A few years ago, I worked with the Super Good Cents program, retro-sealing houses. We had good success with clear silicone. The worst offenses were plumbing penetrations, electric outets and window frames. Nonetheless, we were able to improve thermal/infiltration/moisture performance of these houses several hundred percent in some cases.