Where in Vietnam? It appears that both HAN and SGN have duty free on arrival in their international arrivals hall but I can't find any online shops like the Frankfurt one so have no idea on pricing.

The duty free prices at SIN also seem reasonable, the Veuve is out of stock but is listed at $74SGD which is ~$55 USD right now so a couple bucks more than FRA and in line with US pricing (pre tax).

https://www.ishopchangi.com/en/home#

The point above about taking it in transit is key too. Most of the gates at Changi/SIN do security at the gate so you won't be a sterile passenger while in transit. That means you need to confirm whether a duty free purchase at FRA will be allowed to go through the at gate security checkpoint at SIN.

It appears to be permitted per the FAQs on the Changi website but YMMV: https://www.changiairport.com/en/faq...ecting_flight_