Here's the deal ... it's all over something called the "nine-dash line" -- carving out China's territorial claim. It's located near the South China Sea. Vietnam and other countries are vying for that same territory.

The movie apparently showcases a map that shows nine-dash line is part of China's territory, and that was enough for Vietnam to 86 the film, which is set to debut July 21st.

A rep for the Vietnam Cinema Department put it bluntly ... “We do not grant a license for the American movie Barbie to release in Vietnam because it contains the offending image of the nine-dash line.”