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

Here is Why You Need An Adapter

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

United States Virgin Islands

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

Type A

Type B

Mongolia

In Mongolia, outlets of type C and E are in use.

Type C

Type E