answered about 18 months ago by grubby
last minute is often expensive, and Vietnam airlines seems to always be expensive. You might be better off using a travel agent than doing it yourself.
I don't have anyone in particular to recommend, but perhaps someone else does.
I was pretty sure that Air France was a good option so I had a look for you:
Leaving 20th and returning 28th would cost you 1,133.71 USD including taxes, not cheap but better than VN Airlines.
You could always look into flying Air Asia to Bangkok and getting a cheaper flight from there...