How to Gift iTunes Songs from United States to friend in Vietnam
There is a Vietnamese iTunes Store that your friend (or you) can use directly. The only way for you to gift her a song is under one of the following scenarios:


You set up an Apple ID on the Vietnamese store and use that ID to gift her the song
Your friend sets up an Apple ID on the US store and you gift her the song to that ID


Both of the above are possible, but only you and she can decide whether it's worth the effort (while it's not hard to do, it's not exactly convenient). There is also the possibility that what you wanted to gift her is not available on the Vietnamese store.

My recommendation would be to contact Apple directly, probably best via this form, and explain what you wanted to do and ask them for advice about it. It may take them a couple of days to respond, but they will usually come up with a resolution (whether it is a credit on your account or some way of your friend being able to access it).