How was vietnam able to become and remain autonomous despite china's power and dynamism?

Throughout history, Vietnam was not totally free from China’s power and dynamism, having been under Chinese domination for different, interrupted time periods, the earliest being in 111 BC. Being a strategic southern territory, the Chinese used now-Vietnamese territory to launch expeditions to Southeast Asian states. The last Chinese occupation of Vietnam ended in 1427, with the Chinese Ming Dynasty accepting Vietnam as an independent, yet tributary state. 

20 years ago

explanation:

answer; people where able to travel across the u.s.a faster than any other mode of transportation,travel time was about a week,from coast to coast