According to this site, some were turned over to the South Vietnamese Army. Maybe those were the ones who were eaten?

http://www.uswardogs.org/id32.html
If you google Vietnam War dogs, no quotes, lots of hits:

http://community-2.webtv.net/Hahn-50thAP-K9/K9History6/

What happens to war dogs after a war? See here:
http://dogsinthenews.com/issues/0202...es/020227a.htm

Now, I am not sure that tour guide was accurate. Hmmmmm........