Except (and I know this is getting awfully technical) why didn't something to do with the massive slime flow show up prior to Vigo's appearance in 1989, or for that matter… why'd things come to a head in 1989 at all?
I feel there's more things arguing in Vigo's favour to make the argument of him tapping into the slime flow convincing:
1) The Pneumatic Transit tunnels the slime was primarily flowing in prior to the “Flip City” sequence flow directly to the museum of Art.
2) Vigo directly affected the slime, transforming Dana's bathtub into a monster trying to snatch her baby, control her baby buggy and seal off the museum from outside contact, as well as create the ghostly nanny duplicate of Janosz.
3) None of the slime had shown up in the city prior to the events of Ghostbusters II, and I'm sure if there'd been as significant flow beneath Manhattan that was only generated by negative human emotion, they would've been swimming in it long before the start of the 1980s. The city was drowning in negative emotions, no doubt… but it took Vigo to turn all that anger into something physically tangible, which in turn gave him the energy to manifest more and more, until he had enough to give him limited manifestation outside of his portrait in lieu of loosing the chance to take control of Oscar's body.
I propose that Vigo generated the slime by feeding off of the negative human emotions, with the significant results starting to form once he'd been dragged up from storage. He'd already created enough to start causing trouble by the start of the film's events, and as the final week/s* of December 1989 played out, the flow had grown even more to create the river seen in Van Horne by December 30th (when the guys were arrested).
*Remember, when we first see Dana, it's not even Christmas Day yet (as shown by someone carrying an undecorated Christmas tree home), no Christmas decorations at the birthday party, and the University not having broken up for the holidays). Plus it's usually accepted that the montages span a week or two.
I'm not sure what interpretation should be taken from the river as seen in the portraits of Vigo, but it should be noted that it appears to be flowing in the corridor of a ruined castle, rather than the abandoned pneumatic transit tunnels.