Reading through all this debate, I'll be honest, I have no idea why it's assumed that apparitions would even be the best way to start looking for another of Aetheric Vitae.
Given Mathilde's initial conclusion during her research, it seems it could be restated that she's of the current theory this is something that happens when the Aether spills over into reality without reality being damaged or degraded in the process.
So, rather than asking "how do we find more apparitions to do this with" I feel it might be instead asked "How do you breach that divide without invoking any portal or apparition at all?" And updates like us learning about the Glittering Realm, the Souls of Forests, or even the Grey College give solid hints in that direction, IMO.
For instance, we had an option back when fighting Drycha to pursue her into the Dreaming Wood. Perhaps we might attempt to enter and thereafter seek to bring something out of such a realm and see what happens to it. She's seen Aetheric Vitae decay enough times that any such trip might be informative, even if we cannot immediately find a means using a level of magic exposure Mathilde's experiments have shown to be safe for direct contact with AV.