Calling Vietnam with FaceTime

In my experience, the most reliable way of using FaceTime is to make a Contact entry for the other person. Encode the phone number as +84xxxxxxxx, and enter the person's FaceTime email address if you can learn it. When you press "Done", after a few moments the FaceTime connectivity line should show up on that person's Contact page.

To call that person, open FaceTime, press + for a new conversation, and start typing the person's name. Choose any entry that is blue in color to make the call. If your known FaceTime correspondent's phone and email are grey instead of blue, that indicates that the person is temporarily unreachable (out of range, overnight Do Not Disturb, etc.).

Is the person you're trying to call an iPhone user with FaceTime enabled?

Have you tried contacting them via their email address instead of the number? Depending on the circumstances, their number may not be enabled to receive FaceTime calls.