So we just had it come up that Arcane Vassal seems to interact a bit weirdly with some of the realm spells. Specifically, vassal means you “measure distance and visibility from the vassal” only. It doesn’t make the vassal into the caster; this is good, because it can keep our Slann out of unbinding range. But for Spells like ‘Molten Gaze’ (Realm of Fire), we measure the distance from the vassal and then are instructed to “Draw a line from the caster to that point”. Rules as written, this has to be from the Slann. This can make these spells much more difficult to use without hitting our own! And can also result in a line running a LONG way... Have I missed something?
This is the current rules interaction. And it be can extremely useful. Especially when you consider LoSaT for the purposes of creating lines that run down the enemy's flanks.
That was our conclusion too. Without an FAQ, this seems to be how it works. With the right placement, a Balewind and an Astrolith in range of your Slann, you can have a (JUST short of, if you don’t want to hit your vassal) 41” line of D3 mortal wounds.