Was Spain in Civ4 vanilla? I know it wasn't in Civ3 vanilla.
Yes, it was.
You can quite reasonably argue though that adding a civilisation in Civ V is a lot more time-consuming than IV; trying to work out and then balance the unique abilities is a lot more difficult than picking the traits from Civ IV.
Then add to that the extraordinarily complex art in V; that must take a long time in itself.
I think I saw referenced somewhere the idea that the expansion has taken as long as it has in part because of the time involved in creating the art.