William the duke of Normandy was a vassal of the french king.

William the king of England wasn't.

Reality isn't as neat and tidy as a Crusader Kings game.

Take a look at this answer from /u/khosikulu

https://www.reddit.com/r/AskHistorians/comments/1hls0i/if_william_of_normandy_was_a_vassal_of_philip_i/?

And this one from /u/Gadarn

https://www.reddit.com/r/AskHistorians/comments/21a8wm/how_did_the_french_crown_react_when_william_the/

And this one from /u/agentdcf

https://www.reddit.com/r/AskHistorians/comments/3xzawd/why_did_the_king_of_france_allow_his_vassal/

And if you have time, you may be interested in this Feudalism AMA

https://www.reddit.com/r/AskHistorians/comments/2bs0rc/ama_feudalism_didnt_exist_the_social_political/