I'd say it's a mistake. Very likely a hold over from an earlier draft of the game when Vincent was still being envisioned as a Occult Detective working in Midgar. He may have been intended to have already joined the party by this point, at which point it would make sense he would be seeing a new weapon. When they scrapped that story idea for him and changed it to the version we know, they may have forgotten about it and probably kept it as a hint of his existence since he got relegated to optional status by that point.