For such broad flight searches, a matrix flight search engine is the optimal solution. You could use for example Google’s Matrix Airfare Search.
I did a simple search for you:
For that search query I used Berlin, Germany and all surrounding airports within 1000 miles as origin and all major airports in Vietnam as destination. The screenshots shows the cheapest flight for all the days between 1. of July and 1. of August. As you can see, the cheapest flight is available for 386€.
When we check the details for this flight we see the following:
Mysteriously, the flight is now only 361€. As you can see the flight departures from Prague and flies to Ho Chi Minh City via Frankfurt. The involved Airlines are Czech Airlines and Vietnam Airlines.
The cheapest non-stop flight from Europe to Vietnam I could find is available for 572€:
It is a non-stop flight from Paris to Ho Chi Minh City with Vietnam Airlines:
So generally, I would recommend that you use such flight search engines to find the cheapest and most convenient route for you.
Viet Nam is mostly accessible via hanoi or Ho Chi Minh City (Saigon).
I know that since France has historical relationships with Viet Nam, there are direct flights from Paris to Hanoi on Vietnam Airlines. They are expensive though. I know there are also flights with Qatar Airways, through Doha. They are getting expensive early though, so you should book early. With a few searches, I found flights through Bangkok, as you suggested. Using Skyscanner, I found that the Cathay has flights to Hanoi (through Hong Kong), for not too much.
Credit:stackoverflow.com‘