The King of England was vassal of the King of France, not of a Duke or Lord of England. There is a difference.
The King of England was also never a vassal of the King of France. The Duke of Normandy, Duke of Gascony, Duke of Aquitaine, Count of Nantes, and various other Lords were all vassals of the King of France. The fact that the King of England happened to be all those Lords at various times did not mean that as the King of England he was in any way a vassal of the King of France.
Battlemaster does not allow nobles to hold positions in multiple realms, thus the situation cannot occur, but it was only in France, not England, that the vassalage took place, and thus isn't analogous because at no point did the Duke of Aquitaine hold the fealty of someone he owed fealty to, because that was his highest (tied for highest) title in the realm of France.