You Do Need power plug adapter while traveling from Vietnam to China.

Here is Why You Need An Adapter

The outlet and plug type used in China are A, C and I and in Vietnam are A, B and C. This is how they look like.

Vietnam

In Vietnam, outlets of type A, B and C are in use.

Type A

Type B

Type C

China

In China, outlets of type A, C and I are in use.

Type A

Type C

Type I