I think this is a result of "visa agreements on EU/Schengen level" vs. "bilateral visa agreements". Why some EU-countries have bilateral agreements and some not

I think this is a result of "visa agreements on EU/Schengen level" vs. "bilateral visa agreements". Why some EU-countries have bilateral agreements and some not could have different reasons, two I can think of (warning: speculating): 1. No need for a bilateral agreement meaning too few people would be affected by this (think of: number of expats/tourists). 2. Historical reasons, e.g. due to colonialism. This could be true for your vietnam example since The Netherlands got colonies on now-Vietnames territory.