Break Vassaltion and begin hostile actions?

[Ozi]

I run around some weird stuff:

I supported Letwhin Far-Seeker, claimant to the Throne of the Nords. I started the Rebellion on his side and claimed the throne for him. I am now his vassal and own several Locations (1 Town, 1 Castle, 1 Village). But i am getting tired figthing for this guy and want to become king of my own country. For that i need to be released of my oath. After doing this i cant attack Sargoth (no option for hostile action)...

Any ideas how to get out of this?

 

[Durandal]

Who owns Sargoth? Are you hostile towards them? You can't start a siege at relations of 0.

 

[Ozi]

Ah ok, that was the key, my relation was too good. Thanks.