If you take Huntersbane it converts “Melee attack-> Ranged thorns for vines” and the description states it “casts thorns” using “spirit thorns” (a spell).
+“Minion Spell Damage” does NOT boost the damage
+“Minion Melee Damage” DOES boost the damage
+% “Increased minion spell damage” DOES boost the damage.
For testing the “minion spell damage”. I observed the range of damage for the vines on some low level mobs. Then I put 10 points into “Eternal Nature” (+40 minion spell damage). The damage range for the vines did not change at all. I also double checked with a Summon Spriggan to make sure eternal nature wasn’t bugged and the spriggan had a big damage boost as should be expected.
For testing the “% increased minion SPELL damage” I equipped a T6 sword with the attribute (it didn’t have any other damage bonuses that would impact vines). The damage increased substantially showing that mod is working correctly.
It looks like for whatever reason the flat damage is staying tied to a “melee” attack while the actual % scaling is tied to a spell.