How do I create a feudal vassal as a tribal ruler?
Solution 1:
I'm pretty sure there are no automatic conversions between tribal and feudal character types or holding types. There are strict conditions for converting either one, and there does not seem to be a path for a tribal character to convert to feudalism if they don't hold any tribal lands: the wiki says you must have a stone hillfort to adopt feudalism.
What you can try is creating a new vassal, the way you would for any newly built or acquired holding that you don't want in your personal demesne.