Never trust Vietnam.

It's never in the interest of superpowers to keep its satellite states stable and strong to challenge them.

Take US for example, it has never worked to keep Central and South American countries stable throughout history. In fact, CIA helped overthrow the democratically elected governments of several Latino countries when they threatened US interests in the area, casting them into turmoils. US also support theocratic Saudi Arabia since it was founded, never pressured them to democratize.

North Korea and the military-rule Myanmar are two best examples of ‘ideal’ vassal state for China: authoritarian and poor, hence easy to control. Cambodia and Laos are also good vassal states for China, and are being used against Vietnam.

Vietnam has a long history of rebelling against Chinese rule. During the imperial days, Vietnam never failed to rebel whenever the Central government of Imperial China collapsed. In modern times, Communist Vietnam quickly sided with Soviet Union against China right after the US-Vietnam war ended. And after Soviet Union collapsed, Vietnam quickly allied itself with the US against China. One solid conclusion Chinese learnt from the history: Never trust Vietnam.