The Iron Vrykul you see in Halls of Lightning, etc. are actually creations of Loken. The Halls of Stone and Lightning serve as forges to create them. In the entry to the Halls of Lightning, if you look down off the walkways leading up to Bjarngrim, you will see rows and rows of the in stasis, waiting to be awakened. Whether or not they are meant to be similar to the original Vrykul before they were exposed to the Curse of Flesh ( a la Siamat's gift to the Neferset) I don't think is ever directly addressed. The original Vrykul's origin is implied in the exchange between Brann and The Tribunal of Ages in the Halls of Stone...you can run the event or look up the transcript on Wowpedia. But regardless of whether they started out as metal or not, they were still created by the Titans, thereby making Humans descended from Titan creations.

And to address your original question, I don't think Blizzard has directly addressed this, but the Shaman that sends heroes on those spiritwalks to discover the Humans' origin is readily accessible to any hero that steps off the boat in the Fjord. It would be within the realm of reason to assume that any and all heroes coming home from the Northrend campaign would share that story with the people they encountered on the way and it would spread pretty quickly...but that's just my assumption.