Regarding the Vis transformation guidelines of Incantation/Diablerie, does it only transform a single pawn of vis? (Gremlin44's non-hermetic Grimoire certainly seems to suggest so, but s/he's no definitive answer I know.)
The guidelines themselves makes no mention as to the quantity of vis, which seems to indicate (potential) plural to me, as it makes more sense using the vessel of the vis as the target, rather than the vis itself (i.e you taint a tooth containing 3 pawns of Magical Animal vis into vis Sordida).
Off the top of my head, I'd ask what your target is for the 'spell'.
T: Individual => 1 pawn.
T: Group => up to 10 pawns.
T: Circle => every pawns in the circle drawn as the spell is cast.
... no reason to not use T: circle, is there really?
As I said, the vessel of the vis, rather than the vis itself (Vis is not inherently substantial as far as I know, you interact with it through whichever vessel it's in, like the aforementioned Animal Tooth), and with that reasoning, it should be possible to affect multiple pawns with individual, as the vessel can contain any quantity of vis.
Circle target is a good catch though, completely forgot to think about that.
Meaning it's not a very considered opinion just a quick snap thought.
That could certainly be argued, but didn't you really start this thread because you were unsure if it goes?
...and that's why I've never really thought about it much.
We'd usually just go straight for circle.
Same thing with healing spells really.
I don't think I've ever seen a healing ritual cast with T: Ind like the ones in the books. Always T: Circle.
But I believe your rationale actually matches the canon spells better. Look at Gather the Essence of the Beast, for example. There is one body and all the Vis in the body is gathered. It very much looks like the Individual in this case is the container. I'm not saying you cannot work things the other way around, just that this seems to be typical.