You Do Need power plug adapter while traveling from Vietnam to Cocos (Keeling) Islands.

Here is Why You Need An Adapter

The outlet and plug type used in Cocos (Keeling) 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

Cocos (Keeling) Islands

In Cocos (Keeling) Islands, outlets of type I are in use.

Type I