Mark Millar has confirmed in a CBR interview that Matthew Vaughn pulled out of directing the next X-Men film (see "Vaughn Out as Next 'X-Men' Director") in order to direct Secret Service, based on the Millar comic series. The pair decided to fast-track Secret Service after hearing that scripts for three films based on similar ideas were circulating in Hollywood. "So Matthew and I said, '**** this,' Millar said. "We’re not letting anyone steal our ideas."
This confirms that the rumor that Vaughn dropped out of X-Men to direct Star Wars Episode 7 (see "Disney-Lucasfilm Rumor Roundup") was unfounded.
Vaughn had previously indicated he would do another film based on a Millar comic, but had not said which one (see "Vaughn Choosing Millar Comics Film").