I’m a building contractor, but I’ve never worked on adobe houses – I assume that’s what you have. So with that caveat, let me plunge ahead. You’re on the right track when you propose a vapor barrier – heated air seeks colder air, and on it’s way out takes moisture with it – and deposits it on the first vapor barrier it hits – in your case, the latex paint on the exterior face of the adobe. So what you want is a vapor barrier that prevents the water vapor from penetrating the adobe – and you could paint the inside of the bricks with a good latex or even oil-based paint. If you need more insulation than the 8″ thick walls provide and don’t mind losing the look of the bricks as seen from the inside – and don’t mind the work and expense involved in insulating, I’d still suggest you paint the adobe, then install rigid foam insulation – 1-1/2″ Thermax or equivalent is at least R-11 – and that coupled with the 8″ brick should be plenty. Firring out with 2×2’s is ok, but I’d use 2x4s installed flat – you’ll get a better surface to sheetrock to, and less spitting of the firring lumber. And if it were my house, I’d consider real plaster over Rocklath or Blueboard – or colored veneer plaster over the same substrate – it would be more in keeping with the style of the house. As for the outside, I’m not sure that paint is the only way to waterproof the brick. If it’s natural adobe, you’d lose that beautiful color and texture, and I’d consider a matte finish clear waterproofing like Glaze and Seals Natural Look Clear Sealer – but there are probably many similar products – just don’t get anything shiny. You might have to re-apply periodically, but it’s a lot easier than repainting.
Depending upon where you are I would suggest either that you study the methods used in the adobe buildings of the Southwest U.S. Or if you are in the U.K. Or places like the Pacific Northwest part of the U.S. The “COB” structures made of mud and straw that have stood the test of time. Some Earthen buildings have been in used for hundreds of years. In a mud wall that is heated on one side there will be a temperature gradient within the wall.