das betrifft fast alle Länder in der EU. Da wollen die meisten einen Wohnsitznachweis. Daher, wie du schon sagtest, einfach nach Vietnam fliegen, den Spaß abhandeln und dann direkt rüber nach Thailand. Als PT ja easy machbar.
on ED Visa it was a problem to leave the country while you should be "at language class". So they assumed that you weren't actually going to class, but were just using your visa to stay longer as a tourist. That often led to them cancelling the visa. But thats not a problem with DTV.