You Do Need power plug adapter while traveling from Vietnam to Bhutan.

Here is Why You Need An Adapter

The outlet and plug type used in Bhutan are C, D and G and in Vietnam are A, B and C. This is how they look like.

Vietnam

In Vietnam, outlets of type A, B and C are in use.

Type A

Type B

Type C

Bhutan

In Bhutan, outlets of type C, D and G are in use.

Type C

Type D

Type G