score:4
This website contains all the pertinent information. Specifically, it says that while your friend as to apply for the visa at the German embassy in Vietnam, your "Verpflichtungserklärung" should be submitted to the "Ausländerbehörden" in your hometown. Presumably it will then contact the embassy.
But there looms a bigger problem:
Die Auslandsvertretungen müssen zudem insbesondere zur "Rückkehrbereitschaft" und "Rückkehrmöglichkeit" des Reisenden eine positive Prognose abgeben. Auch hierzu ist der Nachweis ggfs. durch geeignete Dokumente zu erbringen.
This is not something you can influence - basically, your friend has to convince the German embassy that he will return to Vietnam. Typically they look for things like family ties (especially marriage) and a steady job.
Upvote:1
Yes. You need to provide all necessary information to prove that you are capable of covering all expenses for your friend's travel. The documents must be sent to your Vietnamese friends for his visa application. As I know, this is the only way. There's no other option.