Unfortunately, we don't know exactly how it was reported in canon. :c I assume that he was officially declared missing, presumed dead. Unofficially... I imagine Veld and the other turks at the time probably had their own ideas and suspicions, but whether any of that made it's way into any documentation Kunsel might've seen, who knows? Veld & Player Turk from Before Crisis do find Vincent and talk to him, and he helps them take out a few enemies, but somehow I doubt that made it into official reports, either.

So... Kunsel is free to know as much or as little as you'd like! hahaha maybe the disappearance of that one Turk thirty years ago plus the sheer creepiness of the place sparked rumors of the ShinRa Manor being haunted or maybe other rumors / urban legends circulate. Who knows? c: