My apologies if this has been asked before, but I've been wondering this since the Vecna Dossier came out. While the dossier says that the statblock presented is for a Vecna from before Kas betrayed him, he has none of the benefits of his eye or hand. Additionally, the adventure that accompanied the dossier's release shows Vecna missing his eye and hand, but nevertheless his stats remain mostly the same. So does the current statblock for Vecna assume that he just has the eye and hand or is he still missing them and could theoretically get more challenging if he were to get them back?

He still has both of them in the current 5e version of Vecna. It is the version of him from before Kas cut them off, so they're not magical items yet, so Vecna wouldn't get the benefits of those magic items. 

I haven't read the adventure, so I can't speak about that. However, purely from the Dossier, he still has both his eye and hand and does not gain the benefits from the magic item versions of them because they haven't become magic items yet.