You Do Need power plug adapter while traveling from Oman 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 Oman is G. This is how they look like.

Oman

In Oman, outlets of type G are in use.

Type G

Vatican City

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

Type C

Type F

Type L