The reason that Vaughn dropped out of Days of Future Past is rumored to be because he wanted to do Mark Millar's Secret Service. Apparently there were other projects in development that were using the same concept, and Millar and Vaughn wanted to get the movie made before other folks used the idea. At least, that is the word according to Millar. Also, Vaughn dropped out of Thor, X3, and First Class (before coming back to it). So dropping out of movies is not unusual for him.