Is there any policy on how much U.S. dollars a person can exchange when traveling to Vietnam?

The requirement is US$5,000.

The Chinese Embassy in Vietnam reminds you to understand and abide by Vietnam's relevant customs regulations when entering/exiting Vietnam (South), and avoid carrying large amounts of cash, such as carrying foreign currency cash exceeding US$5,000 or other equivalent foreign currencies, or If you carry more than 15 million Vietnamese dong in cash, you must declare it to the customs at the entry and exit ports to avoid unnecessary losses and even serious legal consequences.