For what it is worth, I would not think [name]Van[/name] was a Vietnamese name. It sounds Anglo [name]Saxon[/name] to me or Nordic to me.
And no that does not mean all your kids should have Vietnamese last names if that’s not what you want. But I would make that clear with your husband now as a condition to “giving” him the [name]Van[/name].
I would tell him you know how much it means to him and because of this you understand and will do this. But that means you get more say in future names as a compromise.