You Do Need power plug adapter while traveling from Fiji to United States Virgin Islands.

Here is Why You Need An Adapter

The outlet and plug type used in United States Virgin Islands are A and B and in Fiji is I. This is how they look like.

Fiji

In Fiji, outlets of type I are in use.

Type I

United States Virgin Islands

In United States Virgin Islands, outlets of type A and B are in use.

Type A

Type B