I may be operating under incorrect assumptions but my impression is that the migration era Germanic tribes (Frank’s, Visigoths, etc.) kept a separate legal system for the Germanic people alongside the remnants of the Roman legal system.
Firstly is this correct? More broadly was there a legal distinction made between the tribal people and the pre-existing Roman populations of these states?
Secondly, if this distinction existed, why did it exist? The Roman Empire had emperors of a variety of different origins but those emperors do not seem to have changed the legal status of their ethnic groups. I am wondering why the Germanic kings felt more distinct from the Roman population than the Roman generals who often usurped the Roman state.
I believe that in recent years historians have been moving away from the idea that there were two separate legal system in the successor kingdoms, one for the pre-existing population and one for the newly settled 'barbarian' population. Firstly, it must be said that the information on laws to begin with is very scarce and can be difficult to work with. We have man law codices from the successor kingdoms, but the naturally offer only a snapshot of the law at the time they were published, even though law is a constantly evolving thing. The codices were also all created at different times, places, by different people, and with various unknowable intentions - they were not something as common as law codes today. So any assumptions must be made hesitantly.
The 'barbarian' laws that we have today did exist alongside other laws, but that had actually always been the Roman legal system. Any given city in the Empire could have its own vulgar law code that may have even pre-dated the city's incorporation into the Empire, and that would have addressed issues that the inhabitants of the city felt needed codifying. The citizens of this city, which meant practically everyone after 212 CE, would have also been subject to Roman law issues from the imperial center and the emperors.
In the successor kingdoms, it is now generally believed that the Roman laws that remained provided the basic legislation and legal framework of the kingdom, and the newer 'barbarian' legal codices would fill in the gaps and the areas that Roman law did not address, especially the interactions between the new 'barbarian' settlers and rulers and their Roman subjects. This isn't to say that everyone had legal equality; different laws applied to different groups, such as women, men, slaves, freedmen, etc. Eventually Roman men would even be legal dependents of Frankish men within the Merovingian kingdom. There just wasn't too entirely separate and distinct law codices for Romans and non-Romans.