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

Here is Why You Need An Adapter

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

Cayman Islands

In Cayman Islands, outlets of type A and B are in use.

Type A

Type B

Vietnam

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

Type A

Type B

Type C