I did not. I am sure I had 100% warscore. There is a chance I misclicked and sent White Peace instead of Enforce Demands, but I do not think so. I think it is because he was now a duchy level title, same as I, so he could not be my vassal.
Edit: Oops, already said.
Well, why does it offer these weak claims on duchies when I am duke level then? I assumed I would get the duchy title and the guy would get the lower titles.
It is handy for disposing of people you don't like and put someone friendly on the throne or for weakening other realms or spreading your dynasty. I will quite happily spread my family inside a realm if I'm a vassal even if they would be independent from me.
If you want to gain territory then you should look to your character's claims or holy wars. The latter I know can be part blocked by other realms besieging the target resulting in you only gaining baronies or just the county capital.