Reallty guys, I loved FFXI as much as you, but also, let's be realistic here (and perhaps brutally so). This is not going to be travelling to the literal Vana'diel and doing Dynamis or Dooms Parties in HD or even having literal FFXI NPCs like Prishe or Lion appearing - it's going to be a series of battles against FFXIV enemies in a FFXIV specific storyline with a few FFXI bosses and location names thrown in for fanservice, because we've already had this (Eureka, Alzadaal's Legacy). And this is because Vana'diel has already been established (and stated by the development team in fact), as a seperate world and universe from Hydaelyn and it's Shards, which is extremely difficult to get to, and we've already had several Vana'dielan residents appear here temporarily (Shantotto, Iroha).
I'm calling it now, 'Shadowlord' is simply going to turn out to be a roegadyn pirate corrupted by a Voidsent, and 'Castle Zvahl' is going to be a Turalian ruin overrun by said Voidsent.
Just temper your expectations, because I can guarantee you're going to be disappointed otherwise as it simply will not be what you're expecting (I mean, look at all the complaints about Myths of the Realm with how that story turned out).