Well yes I know all that Olaus Petrus
That is why I had placed quotations around the word "vassal" before King of Scotland. I recognize that there are petty kingships and greater kingships, but that flux is what CK is all about. A king may become a petty king if he lost a war or lost influence, exposing his position and opening himself to become effectively mediatized and subsumed as his power waned. Though later he, or his successors, may regain prestige and reassert their position as a "sovereign realm". But at any rate, the fact that there is still a "centuries long dispute between English and Scottish historians;" (well in this case a Welsh historian Professor John Davies of the Uni of Wales who asserts that the Welsh princes were as much a vassal to the Angivine Kings as was the king of Scotland.) serves to illustrate that there was indeed a flux in releative authority regarding the Scotland and England, and much room for argument either way.
Would the King of France vassalize to the Emperor? Well, possibly a diminished France might. If for instance Aquitania had been fully independent, or England were fully able to integrate the Aquitaine into its realm, possibly the kingship of France at that time... weakened as it was- possibly for generations... may have vassalized to Germany in certain scenarios.
The princes of Wales continued to assert their kingly rank though accepted the mediatized title of prince, as attested to in the 12th century "History of Gruffydd ap Cynan", so in this regard it was a kingly rank mediatized to a higher kingship. Professor Lloyd states that the biography of Gruffydd ap Cynan was a declaration that the house of Aberffraw owed nothing to the kings of England for their kingly position, even if at times occasional homage was given to the English king.
My point was to offer a matrix that would emulate this relationship, and the ability for the king of England to vassalize the king of Scotland, or more likely the king of Wales, would emulate that scenario nicely. For instance, perhaps a vassalized king would lose the "king" title and become a "prince" until he is independent, but this is CKII material.