I'm talking about 900-1200/1300 Both countries are similar, with the Carolingian Empire and Charlemagne as their mutual link/starting point. How exactly did they part ways so differently? Of how much importance were the slaves being a threat in the west for the germans in this?
In the title I meant France under the rule of a king, while Germany was under the rule of an Emperor and many princes btw. Were french vassals equal in power to a german princes?
It depends when you mean. In the middle ages, the idea of a figurehead (The emperor) ruling over lots of underlings (The German and Italian Princes) is pretty common. The French kingdom was divided into many vassal states which functioned very much like the HRE, at some points, some of those vassal states were arguably more powerful than the French Crown. In fact, right up until the edict of Moulins, which made the king slightly more sovereign, it could be said that France was no more centralised than the HRE.
As to why the HRE failed to centralise when the French and Spanish monarchies succeeded in the early modern area, there are countless theories. One of my preferred theories is that France centralised as a direct result of the constant threat of war with the English. England centralised incredibly quickly and as such was far more powerful per person than any state in Europe, and the French soon realised this and by the start of the HYW were getting their collective arses wiped up and down the floor. The constant threat from the outside really forced the French vassals to be a little more subservient to the king, and when they were, they started pushing back the English. By the 1500s the French Crown started to think 'That centralisation thing really saved our lives' and so made positive steps to increase their efforts, with the vassal princes more likely to accept because of their need for protection from the potential of invasion from Britain or Spain.
The HRE was simply too big to have a serious rival in the continent and so the emperor wasn't under as much pressure to centralise, by the time the HRE had proper enemies like France and Turkey, it was too late and independence was so deeply entrenched in the Germans and Italians (Who were pretty much free from the HRE at this point anyway.) and so would have violently resisted centralisation. Then the TYW tore the HRE apart and any chance of the Empire regaining its power and centralisation were gone forever. So basically, the HRE waited too long to try centralise the entire realm, and so collapsed instead.
That's just my favourite theory though, the others range from believing the black death wasn't as bad in Germany so princes didn't need help from the emperor, to the Germans being intrinsically more warlike in that period!