Vaughn isn't necessarily unreliable at all. Layer Cake and Stardust were both good, solid movies. Though I agree with X-maniac's general assessment of Stardust. He directed both of those along with producing Lock Stock and Snatch.
We still aren't totally clear on why he left X-men 3.
However, this is not the first time this happened. Many directors have been attached to movies but then they eventually leave if nothing is happening. Look at Cassavettes with Iron Man which turned out good for ALL OF US.
Also, want to give big time credit for X-maniac. I knew I forgot to mention that he already was talking about this in his blog BEFORE Hollywood Reporter. X-maniac totally called this.
You should all read X-maniac's blog as well. It is very good stuff. I think he makes a good point that Vaughn more than likely wants to stay away from bigger budget material now and stick with lower budget, darker, edgier stuff.