Vaughn seems very confident that a sequel to First Class will happen, and why shouldn't he be? Although the official reviews will not hit until tomorrow, the reactions have been extremely positive so far. In the article below you can read about his plans for a (possible) opening scene, now speaking exclusively to Movieweb he reveals that he would be bringing in a new character too, but only one!. Oh, and while talking about this character Vaughn reveals a pretty big SPOILER: about another one from the movie..
"We will only have one more new character. I won't say who he...I won't say who he or she is! But we will only be bringing one more new character in. Because, I think, as Professor X is in a wheelchair, Magneto needs to have a nemesis he can fight with. Someone that will be his equal. I know who it is. It would be nice if I could say something, but I can't, mate!
Now, from this we can deduce that said character would be working WITH Xavier, and be powerful enough to go toe to toe with Magneto. We know it can't be Wolverine(unless there is some SERIOUS retconning done). So who? Guesses below! Vaughn also doesn't rule out the possibility of either Ian McKellen or Patrick Stewart appearing for flash forward scenes, saying..
"I have worked with Sir Ian McKellen before, and he is a lovely guy. I would love to work with him again. So, never say never!"
Of course, all of this is subject to change, a sequel is likely but who knows what direction it will be taking by the time it goes into production.