You can visit or call any currency exchange or forex shop that is authorised under RBI. Avoid banks because the exchange rates are slightly higher. Also don't buy from the first place that you enter. Check the rates of different shops and then buy. P.S. - All forex agencies won't sell Dong since the value of Dong is lower than INR and they might try to coax you into buying USD. Don't fall for that. It will lead to double conversion and you will lose money. Its better to use Dong than USD since its local, although USD is also accepted there.
Start by calling all the agencies to check the rates.
You could exchange at a reliable source or person you know or you get to know through a trustworthy person . Second option exchange in the airports you commute. There again without prior info about the exchange rates don’t try to exchange for there are chances of losing money. Third go for shops which are authorised by RBI.