I was looking at VN-MY flights and it appears that VN has introduced "Economy Super Lite" fares (booking class: G/P) for flights between VN and KUL, CGK, SIN, and BKK since 20 May 2019.
If you accidentally bought a HBO (hand-baggage only) fare, the cost to check-in a bag is pretty exorbitant:
0-5kg: USD $20
5-10kg: USD $40
The up fare from Basic/Super Lite fare to one that includes baggage is only USD $30.