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

Here is Why You Need An Adapter

The outlet and plug type used in Denmark are C, E, F and K 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

Denmark

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

Type C

Type E

Type F

Type K