The debate regarding vapor barriers is a long, well worn one.
The "older" thought process was that moisture is on the exterior of the property in the form of rain, snow, humidity etc...etc...(humidity controlled internally by AC) and internally excessive moisture isn't/shouldn't be there.
So we use a moisture barrier on the exterior side of the wall to "deflect" (want of a better word) moisture away from studs, insulation and the like.
Now, this is where it gets interesting:
The "new" thought process which has a lot of scientific proof and thought behind it and is in the process now of being reviewed by the IRC (God's of Code
) is, that the moisture barriers are really a Vapor Diffusion Retarder (VDR), that moisture is created during winter on the warm side (interior) and is conveyed to the exterior by the simple physics of cold and warm) and visa versa in summer. Think of the moisture in a home in winter heated to 77 degrees, while outside it's 35 degrees (think about the moisture that will build up on an non air proof window) this moisture is pulled through the drywall and into the wall cavity. Moisture travels in air and if the wall is not "AIR PROOF" then moisture goes where the air goes. Almost all standard vapor barriers are not "air proof" thus allowing moisture into wall cavities, therefore a vapor barrier doesn't effectively stop moisture (though it obviously goes a way towards helping).
Stucco finishes should be sealed either with a waterproof additive and/or sealing coats on the exterior to stop water penetration.
I've attached an article that explains it in depth, it makes for very interesting reading on the future of "moisture barriers"
Understanding Vapor Barriers - Building Science - EcoHome Magazine