I find that when I expand all kinds of counts will want to swear directly to me. Sometimes they'll break from their dukes to do so. This increases the number of vassals greatly.
On the other hand, if you claim all titles you conquer and give them to your dynasty your badboy will go through the roof.
If you hand out duke titles after the counts in the area swear fealty to you, all the counts in the duchy area should become vassals of your new duke.