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

Here is Why You Need An Adapter

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

Cuba

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

Type A

Type B

Type C

Type L

Vietnam

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

Type A

Type B

Type C