I don't know KP, Vigo creating the slime just doesn't make sense to me.
Lets start with Vigo's known abilities (while trapped in the painting)… He can posses multiple people (but not necessarily control them simultaneously) provided that they've made eye contact with the painting (Janosz and Ray), and control them well beyond the confines of the museum, he can shoot energy bolts from his eyes, and he can cause spontaneous combustion (at least within range of people he's possessed). Ok, that seems fairly powerful, but is he powerful enough to cast a spell that effects the majority of the city (i.e. the slime)?
First of all, if he's capable of the above powers, what has he been waiting for? Why has he waited centuries to return to life now? The painting hasn't been crated up since it's creation, someone had to discover it and eventually ship it to the US, so why hasn't he been possessing and manipulating people since day one? Which leads too…
What would he need the slime for anyway? Vigo's goal is to find a host body so that he can be reborn… so where does the slime specifically come in? The possessed Janosz delivered Oscar to Vigo, not the slime. If the slime is so important to Vigo's goals why wait until now to create it (remember, he's had centuries, but not in the same location, so if he created the slime in New York it was a more recent event). He doesn't need the slime to spawn minions… again, he can posses people.
Why didn't he just cut-to-the-chase and posses Dana? She's had direct eye contact and prolonged exposure to the painting… Why didn't he posses Peter alongside Ray? Why didn't he posses the entire museum staff?
Why even fixate on Dana and Oscar?
See to me, Vigo's actions are not that of a powerful mad genius whose been planning his return for centuries… instead, they strike me as acts of weak desperation, of sudden opportunity.
I propose that the slime is a natural occurrence, that probably got a severely major boost when Gozer came to town (just as the spiritual activity originally had). The Gozer incident undoubtedly caused extreme turmoil within the city, and seeing as the containment unit had blown, and the GB's went out of business shortly after, there was probably a large amount of negative spiritual energy in the air years after the fact (all of which ultimately fuelled the slime).
The slime has the ability to make spirits manifest (the Scoleri Bros., Titanic, etc.)… Vigo's spirit was in some part attached to his painting but dormant. The growing presence of the slime awakened his spirit and he immediately began searching for a way to escape and be reborn, but he was still relatively weak.
Vigo wanted a young body to be reborn into and as luck would have it Dana (a staff member of the museum… i.e. within easy reach) had Oscar. Unfortunately, Dana's willpower would be too strong for Vigo in his weakened state, so he instead waited for a safe opportunity to posses Janosz (who was emotionally fragile from being recently rejected by Dana).
You'll recall that the first possession didn't fully take as Joanosh was still able to question Vigo, forcing Vigo to give it a second go (the energy bolts). Later Vigo would even prey on Jonosh's weaknesses (his desire for Dana) to ensure his loyalty.
Since he had awakened, Vigo had been slowly drawing energy from the slime but was also fueling it as he was such a purely negative spirit. The slime probably absorbed Vigo's desire to use Oscar as a host and began to independently reach out to the baby (the runaway carriage, and the bathtub… because honestly, just how would Vigo benefit from either of those incidences… by slowly pulling Oscar down the drain? And again, if he could just abduct babies with slime, why fixate on Oscar).
Anyway, enter the GB's, where once again Vigo has to slyly posses Ray, who once again probably has the most vulnerable personality of the four GBs.
Vigo continues to draw more and more power from the slime, which is why it flows to the museum and not away from it (“away” being the direction it would flow if Vigo were the source)… or, I suppose, the slime may just be drawn to Vigo… It's not 100% clear if Vigo can manipulate the slime or if the slime acts independently to feed off of and protect the largest negative force in NY city (again, using the slime to capture Oscar doesn't make much sense, and Vigo is never shown directly manipulating the slime).
Yadda yadda yadda, and you know the rest.
None of Vigo's actions really display any real calculation, or a particularly high level of power. I just don't think he was capable of creating the slime.(*winston)