Yet another answer, only slightly different from the other ones.
The case of German is actually interesting, and I'll come back to this later. But first we need definitions.
In theory a vernacular language and a lingua franca are exact opposites. The latter being also termed a vehicular language.
- As you have correctly hinted in your original post, the origin of the phrase vernacular language is that of "the language of the servants".
This is becoming clearer if one remembers that the ruling class in the imperial1 provinces of the Roman Empire was dominated by Latin speaking colons (originally veterans having been awarded a plot of seized land) and by the imperial administrative staff. Cities also abounded in migrants from all corners of the empire who could only use Latin to communicate with each other2. As a result, the indigenous languages of the colonised populations were usually overshadowed by Latin and could only survive by being used by such classes as the slaves and the servants.
Latin instead, in these circumstances was the vehicular language because it allowed various non native speakers populations to communicate without having to learn each and every one of the hundreds of languages spoken in the empire. The typical difference between a point to point topology and a hub topology.
- To figure out the origin of the lingua franca, one has to travel another millennium in time and imagine that the Mediterranean was crisscrossed by a multitude of ships and that the crusaders had taken the control of the major Islands and several important ports at least on the northern rim. The vehicular language was therefore a kind of pidgin evolved from Old French: the so called Lingua Franca – "language of the Franks". On the southern rim, the lingua franca was still spoken by the “roumi” (roman) slaves and their integrated descendants.
Now to your reference to German. There are indeed a number of minorities in eastern Europe who speak German because of their origin (Hanseatic League, Teutonic Knights, Peter the Great guests... to name only the most well known cases) and an even greater number of people who speak very well German simply because they have learned it (possibly as migrants in Germany or because they do business in German). This is particularly true in the Balkans, in Turkey, in Poland in the Czech Republic etc...
Just remember how, before the advent of the Euro, many countries had two currencies: their own and the Deutsch Mark.
So in this sense yes, German has all the characteristics of a lingua franca. It is however obviously competing with Russian in the former communist countries.
To label German a vernacular language however, one has to go more than 15 century back in time.
As opposed to the senatorial provinces which were the result of "older" conquests (typically made under the Republican era).
2 Or Greek for the eastern half of the Empire.