The Kick-Ass filmmaker stepped down from the director's chair of X-Men: Days of Future Past, sparking rumours he is in line to direct Star Wars: Episode 7.
However, Millar has now confirmed that Vaughn handed the X-Men sequel over to Bryan Singer so he could fast-track the development of the Secret Service adaptation.
Millar, who wrote Secret Service, and Vaughn reportedly learned that there were several rival projects about to enter development, forcing them to move ahead with the film.
"We thought Matthew was going to do X-Men first, but we found out there were actually a few imitators of Secret Service in the works," the scribe told CBR.
"People are always trying to rip you off. Somebody will start to work on a spec screenplay about your idea. There are three other projects I know of now about a James Bond kind of guy taking a street kid and turning him into a spy.
"Three screenplays were already going through Hollywood about this! So Matthew and I said, 'F**k this. We're not letting anyone steal our ideas'."
X-Men: Days of Future Past, starring James McAvoy, Michael Fassbender and Jennifer Lawrence, is scheduled for a July 18, 2014 release date.
Watch a trailer for X-Men: First Class below: