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

Here is Why You Need An Adapter

The outlet and plug type used in Pitcairn Islands is 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

Pitcairn Islands

In Pitcairn Islands, outlets of type I are in use.

Type I