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

Here is Why You Need An Adapter

The outlet and plug type used in South Africa are C, M and N 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

South Africa

In South Africa, outlets of type C, M and N are in use.

Type C

Type M

Type N