If you want to buy Bitcoin (BTC) by Vietnam Dong (VND), the concept here is understood as swapping VND for BTC.
You send: VND.
You receive: BTC.
Step 1: Choose pair VND -> BTC
Please note that "VND Bank transfer" and "VND Balance" are not the same, you can read more here. Basically they are different deposit methods of BitcoinVN exchange.
Step 2: Enter the amount you are willing to buy
As soon as you finish typing on the left box in VND, BitcoinVN will show the amount of BTC you can receive on the right box.
Step 3: Click SWAP button.
Depends on sending and receiving methods, BitcoinVN will show you the next step. Swaps will be done instantly if "Balance" accounts are chosen, eg. VND Balance -> BTC Balance.
In this example, because we choose VND Bank transfer method, BitcoinVN will show you the required banking information to continue payment.
(Optional) Step 4: Finish banking transfer
A sample bank transfer information is shown like below. Please remember to send exactly amount number shown on the form, and any Bitcoin or cryptocurrency-related words should not be mentioned in transfer description.