The original Yue is in modern day Guangdong and guangxi only. They drew the yellow part up to the yangzi which is traditionally chu and Wu areas.This may appears intended to show the original Yue people's geographic area. It's not really land claimed by Vietnam, since there was no Vietnam and not even a certain dynasty that could be said to be Vietnamese until about the 10th century, by which the Yue people had fragmented. So, this is more a map of ethnicities than anything else.
As for the language components, while Yue language contributed a lot to the areas where the people used to occupy, modern Cantonese is more like a creole of Chinese varieties coupled linguistic drift from Tang dialect. And this increases as you go farther South.