Hi Kat, congrats on your fun project and blue boogers. My main question is what climate are you in? Usually, vapor barriers VBs are only required/recommended in the coldest N.American climates.
Your question gets into some pretty heavy building science issues. Tight is great because it means more control of your indoor, conditioned air. VBs are an old-school way of getting tight (air barrier) and they have largely fallen out of favor. Most building code departments will accept a simple latex paint on drywall if there is still something in the code that requires them.
The problem with VBs is that they can create a condensing surface. Breathable (like gore-tex, tyveck, OSB, plywood, #15 felt) is a desirable attribute when it comes to air barriers because it allows moisture in the wall to escape. Depending on who you ask however, one VB is usually OK. Iam assuming your VB is on the interior (between studs and plywood in place of drywall). It would be very dangerous adding a VB on the exterior because moisture might get trapped in the wall.
I think your 15# felt (tar paper) would be fine and personally, I would go for at least two layers and ideally have an airspace between the stucco/plaster coat and felt. The true air gap is a tough detail and the two layers is an easier way of achieving it. Most important are your
flashing details. Moisture rarely gets in the wall from vapor drive although its much higher with porous claddings like stucco (hence the air gap recommendation). Moisture damage is usually the result of poor flashing and when it accumulates faster than it can dry out, the results are bad.
Doing it over, and if climate/code allowed, I would
skip the VB and tape the plywood using it as the secondary air-barrier. The danger of interior VBs are usually in the Winter when warm, humid interior air finds its ways to the colder VB. My advice is to be meticulous eliminating these air paths and consider some
exterior, insulative sheathing which will help raise the temperature of the condensing surfaces inside your building envelope. Your ground up wranglers will do a great job insulating the cavities but the studs and framing members (thermal bridges) will be working hard to rob your heat and lower the temp of the VB.
Hope this helped more than it hurt!