Look for a buy offer on Localbitcoins[1], and simply transact with the person. If you don't like the prices listed there, you can create your own sell offer.
Do this as an absolute last resort. You'll get a terrible price and as mjq said, it can be dangerous. When would you ever broadcast that you'll have a significant amount of money on you and an exact date, time, and location? Never. You can get killed easily over this stuff.
First (and ideally instead) you should find a bitcoin exchange that operates in Vietnam. Coinbase, Xapo, any of them...check their website and see if they serve your area. If so, you're all good. Create an account, link your bank account, transfer your bitcoin stash into this wallet and withdraw.
If you have more specific questions, come talk to us.
Good luck.