We have a number of problems here. First is Flanders cannot be a Kingdom. The Kingdoms in the game (at current patch who knows what the future might bring) are hardcoded, and Flanders is nto one of them. Secondly Flanders cannot have vassals from both Kingdoms. Either Flanders is part of France and all it's vassasls are, two Flanders is part of Germany and all it's vassals are, or Flanders is a vassal of niether and so too are all it's vassals. Three by setting up Flanders as not part of it's lieges royal demise then it is independant, it can wages it's own wars, gain it's own inheritances, even wage war on its liege. So we can give you all you want and still have the historical France HRE boundary (i.e. through the low countries) by having Flanders with it's own Duke. as part of France, and have Imperial Flanders being held by the same Famaly. The CK system cannot model the complexities of someone holding some provinces form one liege and others from an other. The Flemish Dukes may well of been very independant but they still did acknowledge nominal athority from one throne or the other.