Originally Posted by Tokyoite

The problem is convincing the airline taking you to Vietnam that you won't have an issue there. They simply won't let you board, since the QR ticket is a separate ticket and they won't take the risk. Best plan is to get a single entry 3 month visa, or an invitation letter, and simply not use it till the very end (you'll need it again on your return to Vietnam as part of the QR ticket).