Well it's not exactly unheard of for a vassal of one kingdom to be the king of another. From 1066 to whatever date the French finally defeated them, somewhere in the 1300s, I believe, the English King was also a vassal of the French King, and for much of that time was actually one of the biggest land owners in France.
Of course, that was the result of a vassal taking land for himself, not a king becoming a vassal. Was he still the king of Swadia at that point?