If it's any help, you can buy minutes/data from any convenience store.
Just say Vietel 100 (for 100k duong) or 200. They'll give you a receipt with a code.
Use the code on the receipt by calling.
*100*code#
To check your balance
*101#
To buy data from your balance, text to 191
Dmax (1.5 G 120k duong)
MiMaz (600k 70 duong)
Check your data balance by texting kttk to 191