You Do Need power plug adapter while traveling from Niue to Vatican City.

Here is Why You Need An Adapter

The outlet and plug type used in Vatican City are C, F and L and in Niue is I. This is how they look like.

Niue

In Niue, outlets of type I are in use.

Type I

Vatican City

In Vatican City, outlets of type C, F and L are in use.

Type C

Type F

Type L