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

Here is Why You Need An Adapter

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

Cambodia

In Cambodia, outlets of type A, C and G are in use.

Type A

Type C

Type G

Vietnam

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

Type A

Type B

Type C