Folks, no need to be rude to each other. Personally, I wouldn't really call it cheating if he takes the mission, fought an honest war and used console for this particular mission. After all, in a single player game, you only need to be honest and fair to yourself. It may not be a bug, but I'd call it a developer oversight.
I do think the hard-coded event missions should allow vassalization at maximum 100%. I vassalized Scotland in my previous game. I'd rage if I play England for the second time and find out they patched it in the mean time to strengthen Scotland and the vassalization score is now 108%. I'm not a saintly figure and I don't think I'd feel the game is playing itself fair. I heard the OE gets mission to vassalize Algiers which it cannot. I haven't done it myself, but if I got the mission without the previous knowledge that it cannot be done, and used the Subjugation CB like... most casual players who don't visit forums and nerd it up on the wikis etc, I'd feel no less than frustrated.
As an aside, the missions are probably some of the weakest parts of the game as a whole. The Italian Ambition as mentioned before doesn't give any claim and it's basically pointless. Alternatively, 'Kick Aragon out of Italy' thing that Italian nations get doesn't offer any claim. Just a few points of prestige - I'm not saying prestige is bad, but by the time I killed enough Aragonese armies and got the peace my prestige was maxed out already. Huh, great mission. Maybe it could at least reduce the AE to other Italian nations? Nah, I'm just being silly there's no way such a thing could exist.