Charlie was delighted when I showed him his British birth certificate, that I picked up at the British Consulate yesterday morning. Not actually sure if Charlie was British before he got this piece of paper, but he definitely is now.
Next month we will be getting Charlie his British passport and probably a Vietnamese one as well.
Under Vietnamese law it is theoretically illegal to have another nationality in addition to Vietnamese, but the government knows that this happens and doesn't appear to have an issue with it.