If I usurp a King, will all of his vassals swear fealty to me?

I am the King of Ireland, and I've conquered Wales and carved myself a good chunk out of the Kingdom of England. My goal is to drive the English off the island, and form the Empire of Brittania.

Problem is, the remaining English provinces are all ruled by one-province lords, except for the King, who has two (plus one in the Holy Land). Thus, the usual methods of fast expansion, such as smart marriages and duchy claims, are no longer effective, and taking the Kingdom one province at a time is tedious, tiresome, and time consuming.

If I decide work to usurp the King, will all his de jure vassals be transferred to me, or will they become independent?

My guess is that both the vassals and the former King will become independent, and that I may attempt to offer vassalage.

I have no claim to the title, and I don't care about it – in fact, I intend to destroy it as soon as possible. I just want more provinces. Therefore, it is imperative that I get as many vassals as possible in one swoop.

I'm playing an old version of Crusader Kings 2 (v2.0.4).

Solution 1:

When you usurp a title, no land is actually transferred to you from the usurped. The King of England would simply take on his next highest level title as his primary and remain independent, I'd assume as the Duke of Something-or-Othersex. Any lower level vassals will remain underneath him, with any same tier vassals becoming independent.

You can offer vassalage, but as someone of a different culture (I'll also assume your character is Irish culture) it's highly unlikely that they'll accept. Being the de jure liege certainly helps, but that different culture will cost you a -4 (since you belong to a different culture group) to their willingness to accept. Any Dukes will also give you trouble due to "Small Difference in Rank", which is also -3.

In the end, what usurping will likely do is give you easy claims on every county in the de jure kingdom that you can press over time.

Solution 2:

If you usurp it, no. You'll only get that title. Vassals will continue to swear fealty to their loge if possible. If not, because their liege is now the same ranl as them, they will become independent.

You will have a de jure claim on anyone in England which will make some of the counts willing to swear fealty if you ask. If not, you can use this to take their land.