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

Here is Why You Need An Adapter

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

Luxembourg

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

Type C

Type F

Vietnam

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

Type A

Type B

Type C