I have looked into the VNA website (English and Vietnamese language) and could not find any clear explanation either.
I did find that Business Flex is J fare and Business Standard is C fare.
Therefore my own guess is that these are different fare designation with no difference in benefits, similar to Economy class where you will find a wide range of fare from H, K, L.... to N, O and S. Same benefits but different prices.
Or I may be wrong.