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

Tonga

In Tonga, 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