Yeah they're not changing anything with Vulcan. Sinister teased (back in Powers of X #4) that there were more than 3 siblings so this seems to just be paying off on that. Adam-X's story (in X-Men Legends #1-2) as told in-continuity (circa X-Men stories of ~1995-1996) would just take place before the introduction of Gabriel (in X-Men: Deadly Genesis (2006)). Editorial would make sure that nothing in Adam's story contradicts the "later" stories involving Gabriel. There's a good quote from Fabian (in interviews for the upcoming X-Men Legends) where he talks about working within certain boundaries/confines with an in-continuity flashback story vs. the freer reins you'd have with an in-sequence or out-of-continuity story, so he is respecting other established canon.