You're sure you got it right China is 10x of Vietnam GDP per capita? According to world bank data China GDP per capita is
6,807.43 USD while Vietnam 1,910.51 that makes China GDP per capita is around 3.5 of Vietnam, not 10x like you claim. Now compare with GDP per capita of Japan 38,633.71 USD, Japan GDP is about 5.5 of China.
Firstly compare each country start line.
China starts reform in December 1978.
Vietnam starts reform 1986
China 8 years head start.
Secondly, compare each country condition.
China sold out Russia to get economic assistant from U.S., Japan. China has 3 satellites Hong Kong, Macao, Taiwan which really help lifted China economic.
Vietnam warred with China, embargo by U.S., infrastructure largely destroyed.
Take into accounts these things Vietnam is not really far behind China. Now it is up to who has better leadership.