Make my visas more useful

One of my greatest sources of frustration is my inability to travel spontaneously and freely due to my Vietnamese citizenship. However, since I posses both an Schengen Visa by Germany as well as a student visa by the US, I carry a level of credibility and safety that some countries view sufficient to let me enter them. I have so far done so with Mexico and Costa Rica, thus I figured I assemble a list of countries that allow visa holders to make their visas more useful. While this list is not as long as I hoped, I'm sure many of you unfortunate people like me who do not possess the magical Western passport can take advantage of it.