if my slann casts a life spell through an arcane vassal, would I use the vassal or the slann to determine which model can be healed? I think I'm leaning towards the vassal, since I get the feeling that the miscast clarification (that it is resolved on the slann) is there precisely because miscasts are the only part of the casting which takes the position of the slann into consideration. (I'm sorry if this is already discussed elsewhere, search didn't find it)
I'm pretty sure it is totaly opposite of what you think. Part about resolving miscast on the slann is there only because so they could add part about S3 hit for skink priest without sounding like there is no normal miscast. The Slann is a caster or wizard for purpose of all spells he only gets to cast them by using forward arc, LOS and range of a skink priest for a cost of this additional S3 hit in case of miscast.
so, would you say in the same way that if the purple sun started on top of the caster, it would be the slann and not the vassal that got nuked?
Technically I would say yes but practically you can't cast Purple sun through Arcane Vassal because it's not a direct damage or magic missile.