I don't know about Brytenwalda (I just renounced my oath and surrendered my fiefs, then went off to capture another place) but in Warband you can keep your fiefs. All you need to do is renounce your oaths to your king, and then he says "You understand this means you must give up the lands you hold in my name" or some such, and then you have 3 options. One is you say, alright, cool, here, take them back and free me. Another is nevermind, I'll stick with you after all. The third, however, is something along the lines of "I have no intention of giving up my fiefs" which should do what you need.
Mind you, in Warband I've always either taken the first option or rebelled after being refused a fief, so I have no idea what taking the third option would actually do. Nor do I know if it exists in Brytenwalda (wasn't really paying attention). Still, try it out and see.
Edit: Since you weren't actually talking about going through the King's menu, I figured I should add this too in case you didn't know. As I mentioned in passing above, after you capture a fief, you can ask your king for it. If he doesn't give it to you, you should have the option of rebelling, keeping all your fiefs but now being at war with the faction you were a part of.
Of course, you don't keep villages either way unless you have the adjoining castle/town.