I'm assuming that would be the only way a sequel would be made, he and Millar seem tight and on the same page at least with most stuff. The statement I posted above regarding Vaughn was during an interview for "X-Men: First Class" I believe, the interviewer had also asked him about the status of "Kick-Ass 2" of which he gave my comments...I'm paraphrasing of course. He actually added that "no, we're not planning a sequel, if it continues along then we might but I just don't like counting my chickens before the eggs hatched" that was back in April. Chloe Morentz is more enthusiastic and said she would love to reprise her role of "Hit-Girl".
Millar is a bit like me...talks before thinking about it and all that...and I think gets excited about projects he's probably not supposed to talk about it. I'm of the opinion that if Millar has something ready for Vaughn and Goldman to adapt by the end of this year or early next year and Vaughn doesn't take on another project after First Class is finished and Fox doesn't fast track the inevitable sequel, then I think we could have "Kick-Ass 2: Balls to the Wall" for 2012, more likely 2013. That's just pure speculation on my part.