Foreigners are charged different amount compared to locals for the entrance of the Cuchi Tunnel.
Its not only in Cuchi Tunnel but also in some other sites. This stupid idea still survive after so many and many complaints from foreigners and even locals. Last year, I got double charge in My Son Holyland though I m a real VNese because I forgot to speak Vietnamese to the cashiers

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For currency exchange, dont exchange at the airport, the rate there very low. You can exchange at a lot of currency exchange places in the central. Another solution is you exchange first SGD to USD in Singapore and then use USD in VN. Its very easy to exchange USD to VND in VN and you no need to worry about the rate is good or bad. Some restaurant, pub they also accept USD also.
And remember dont give tip to any people: taxi driver, restaurant, hotel ... You spoil them, certainly, except the "sensitive" place like massage spa or pub. Just stick to the money in the bill and round up to 2000 VND ( smaller than that the local people not easy to find the changes for you).
I will be in HCM City around Oct 11 to Oct 18, Nov 22 to 29, Dec 28 to Jan 05. So if you need a guide from a local just pm me

Willing to help