Hmm... I don't THINK so, but I believe the Astrolith does affect the range in 3.0.Can a Slann cast a endless spell through an Arcane Vassal?
Yes. Looking back at the core rules on my app, I think you're right that Arcane Vassal should work.This line from the core rules is what made me believe it might work now and I think it is also why the Astrolith affect the range
"The spell used to summon an endless spell is still a spell for rules purposes."