From the above link (which I couldn't see for looking previously...)
• Transit/Flight connection: In most cases you do not need a visa to enter/re-enter Viet Nam if you do not cross passport control/immigration when catching connecting flights. Please double check carefully with your travel agencies and airlines for specific requirements at your transit airports. This information cannot be confirmed by the Embassy.
If you cannot confirm whether you have to cross passport control/immigration or not, please consider applying for visa to avoid any problem during your transit.