I know that we have it over here, Trang! We have a few Vietnamese coffeehouses and restaurants and one does serve egg coffee and salt coffee. Also, my friend is from Da Nang and is always looking to find new places.
Because you went to a big city, look up a few to see if you can find one. If they don't serve what you want, ask for it. Many ethnic restaurants here will serve off menu if they know how to make it.
Love this post and LOVE that coffeehouse!