In an interview with HitFix, Vaughn said that he already has some thoughts for a future collaboration with the X-Men:
"I've got some ideas for the opening for the next film. I thought it would be fun to open with the Kennedy Assassination, and we reveal that the magic bullet was controlled by Magneto. That would explain the physics of it, and we see that he's pissed off because Kennedy took all the credit for saving the world and mutants weren't even mentioned. And we could go from there, and I've got some fun ideas about what other mutants to bring in. I don't want to tempt fate, though. If the film's a hit, of course I'd be interested. I really enjoyed making it."
Zack Snyder's Watchmen also referenced the Kennedy assassination in its opening moments, but would you be up for seeing how superheroes affected the 1963 event again?
ยท Interview: Director Matthew Vaughn on making 'X-Men First Class' feel fresh [HitFix]