Were there any contacts between the ancient Egyptian, Keralan (South Indian), and Vietnamese civilizations that may have been significant enough for these people to have borrowed elements from each other's mythos?
In Kerala (Malayalam), the most important festival is Onam. King Maveli (or Mahabali) made Kerala better than heaven, and was defeated through fraudulent means. He will come back once an year. He is in netherworld now. While he ruled, everybody was honest, happy and equal. A god (Vamanan) did the deception to defeat the king.
In Vietnam, the story is reversed in the sense that the god (Buddha) uses dishonest means (using a robe which changes its surface area) to get rid of a bad supernatural being. But otherwise, the story is similar.
I did find an article on Tet that says:
The story of Vishnu and Mahabali may have traveled to Vietnam when Hinduism was widely prevalent in the country. There are quite a few ancient Hindu temple complexes in the Southeast Asian country. As is the case in nearby Thailand, Vietnamese believe that Buddha was an avatar of Rama (and hence an avatar of Vishnu). The story of Mahabali probably underwent changes when most of the country adopted Buddhism.