You Do Need power plug adapter while traveling from Oman to Vanuatu.

Here is Why You Need An Adapter

The outlet and plug type used in Vanuatu is I and in Oman is G. This is how they look like.

Oman

In Oman, outlets of type G are in use.

Type G

Vanuatu

In Vanuatu, outlets of type I are in use.

Type I