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

Here is Why You Need An Adapter

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

Nauru

In Nauru, outlets of type I are in use.

Type I

Vietnam

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

Type A

Type B

Type C