If you want to use Delta miles you need to book through Delta.

None of the Delta award partner carriers that could take you to Vietnam - including Korean, China Eastern, China Southern, China Airlines, Vietnam Airlines - is award-bookable online, so you need to call.

Delta allows a stopover and an open jaw on intercontinental awards, so long as the stopover is on a valid routing. Typically (but maybe not exclusively) awards between the U.S. and Asia require a trans-Pacific routing so forget about connecting in Europe.

Buying a VN ticket and upgrading would probably require a full Y fare to use DL miles. That's going to be very expensive.

If you book an award and seek to change it after ticketing, you'll pay $150 per ticket unless you are Platinum or Diamond.