Honestly, I would've considered this match to be inconclusive (or possibly even a victory in Vanilla's favour) if not for one reason: Vanilla Ice can't see while he is attacking in the void.
Being a sphere of space-destroying void that destroys matter is all fine and dandy, but Vanilla Ice literally cannot see while is in that state, essentially making him blind in combat. This limitation forces him to have to stop and look out of his dimension every time in order to see where his opponents is and know the current situation he's in. This limitation is practically the main reason why he lost to Polnareff.
Against a walking, sapient virus like Mercer who would start to infect everything around him with his viral vapours at the beginning of the match (which he has done in Prototype 2, infecting an entire city in a relatively short span of time, and would've infected the city quicker if it weren't for Blackwatch fighting back against the infected and trying to quarantine the infection), Vanilla Ice would literally get infected the moment he open his stand's mouth to peek out of it. The fact that Cream seems to absorb matter into its mouth into an unknown dimension also shows that it doesn't actually erase matter from existence completely, thus Cream would just end up sucking up all the infected gases and get Vanilla Ice himself infected. The fact that the location is an a closed space like Dio's Mansion would make Vanilla's chances of getting infected even higher, especially since an environment with a closed space like that would end up having the entire mansion be filled with infected gases. With something with such a great infection scale like the Blacklight Virus (which should be capable of infecting both living beings and inanimate objects like its predecessor, the Redlight Virus), I doubt even Vanilla's vampiric Regenerationn would save him from that. As the Blacklight Virus that has infected the original Alex Mercer's corpse reconstructed his entire body cell by cell, this shows that the Blacklight Virus should at least be capable of infecting at a cellular level, thus the cellular-level infection should overwhelm Vanilla's potential High-Mid Regenerationn (infecting every single cell in his body). At this point, Vanilla might as well be dealing with a being that is potentially far worse than Pannacotta Fugo's Purple Haze.
Mercer's puddle regenerating feat via crow's biomass should mean that he should have enough biomass to survive and regenerate from Vanilla's blind attacking as an undetectable void-sphere (At this state, I doubt he could annihilate matter whole like he could with Cream's mouth, as he only seems to destroy a sizeable chunk of Polnareff's flesh upon getting a hit), which should allow him to last long enough to infect Vanilla. Or, you know, Mercer can just punch a hole through one of the windows/walls and get out of the building and wait for Vanilla to get infected (which he should be intelligent enough to plan such a strategy, as he has the knowledge and memories of thousands of people, which includes military soldiers and commanders). I'd say Mercer wins this with mid-high difficulty.
