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

Here is Why You Need An Adapter

The outlet and plug type used in Vietnam are A, B and C and in Monaco are C, E and F. This is how they look like.

Monaco

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

Type C

Type E

Type F

Vietnam

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

Type A

Type B

Type C