Vietnam's legal tender is the Vietnamese dong. Although some hotels or large restaurants charge US dollars when checking out, it is a minority after all. It is recommended to use the Vietnamese dong.

At present, the exchange rate between RMB and Vietnamese dong on March 29th, 2119 is around 1:3441. If you exchange it at a local hotel in Da Nang, it is usually around 3,111. If you exchange it with a tour guide, it is usually around 3,111. Some large restaurants or storefronts are exchanged at the exchange rate of around 1:3111, and it is generally not convenient for local banks to exchange RMB for Vietnamese dong.

-The above is what I experienced when I went to Da Nang in June 2118 (the exchange rate was around 3,411 at that time).