Amber or yellow headlights are, and always were, illegal in all european countries, except france, where they used to be mandatory.
In order to pass technical inspections, yellow headlights must be removed, and white ones installed. (You may get away with yellow headlights if they were original to the vehicle, and you register it as historical.)
This does not apply to foglights.
Foglights may be white OR yellow, and can be used with the regular headlights off.
When you look closely at pediod vietnam photos, you will notice that most yellow headlights are just regular white ones with the top half painted yellow.