This is the primary site used to make face-to-face trades with a local seller. You can arrange a meet up and negotiate prices for the Bitcoins. The site also has an added layer of protection for both parties. If you are not comfortable with one on one trades, use Meetup.com to look for a Bitcoin meetup group. You can all then decide to buy bitcoins as a group and learn from other members who have used sellers to buy Bitcoins before. Depending on the seller, you may pay a premium of around 5-10% over the exchange price for a face-to-face trade. You can check the current Bitcoin exchange rates online via http://bitcoin.clarkmoody.com/ before agreeing to the seller’s rate.  You should also ask the seller if they prefer to be paid in cash or via an online payment service. Some seller may allow you to use a PayPal account to pay, though most seller prefer non-reversible cash as payment. A reputable trader will always negotiate the price with you before you meet up. Many will not wait too long to meet up once the price is finalized, in case Bitcoin’s value takes a dramatic shift. Avoid meeting in private homes. You should take all the precautions, especially if you are carrying cash on you to pay the seller for the coins. When you meet the seller face-to-face, you will need to access your Bitcoin wallet via your smartphone, tablet, or laptop. You will also need Internet access to confirm the transaction has gone through. Always check that the Bitcoin has been transferred into your account before you pay the seller.

TL;DR in Vietnamese:
Tìm người bán trên LocalBitcoins. Sử dụng trang Meetup.com để tìm người bán. Thương lượng giá trước khi gặp mặt. Gặp mặt người bán ở nơi công cộng nhiều người qua lại. Truy cập vào ví Bitcoin.