Cement and concrete are alkaline in nature. Alkali
slowly attacks polyvinyl acetate, forming acetic acid, which has a low pH. Cement and concrete cures through hydration, which means the binding of water. This causes the pH of the substance to rise dramatically. Introducing an acid negates that process to some extent, preventing the cement or conrete from binding all the water it needs. It is hydrolysis which gives cement and concrete products strength, and holds them together. Without this process, it would merely be the powder you started with.
Furthermore, the acetic acid will continue to free the water bound through hydrolysis, and that will weaken the bond and/or integrity of the cement or concrete. The effect is accelerated if the material is subjected to moisture, which is more or less always the case.
PVA isn't water resistant. It becomes slightly live when exposed to moisture, and this in combination with the exposure to alkali, accelerates the forming of acetic acid. PVA which is marketed as "waterresistant" or "exterior grade", has additives which makes them water resistant, but they're not alkali-resistant.
I'm hungry, I'll post more later.
Edit: oh yeah:
Ettringite formation over gypsum screed