It is no new plan to give Vega a German or French upper stage because of their expertise with liquid propellant engines. It simply never happened because of the way how ESA is permitted to spend the money.
In simple words: ESA has to spend their money like the member nations pay. That means that the small nations also need to get a chance to lead a bigger project, and not just have Germany, France and UK lead everything. A Ukrainian upper stage was the prize for not one country swapping places with Germany on a project that the other country can not do as well (yet).
Now that the first phase of Vega is done, Germany can really jump on the train for the follow up work, but that is politics again.