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

Here is Why You Need An Adapter

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

Belgium

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

Type C

Type E

Vietnam

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

Type A

Type B

Type C