2023. hearthstone players STILL don't know how basic mechanics works.
Let me ask you a question.
You have Wild Pyromancer on board. And you play Yogg-Saron, Hope's End How many instances of 1 dmg will Wild pyro deal? Because this is the same situation. Something casts spell for you, and you expect to get benefits
It's not the same situation. By that logic, why do I, the player, get overloaded for Overload spells cast by my Yogg? Or better yet, why do I get overloaded for spells cast BY MY Nagaling?
And if you don't like that example, here's a fun fact: A Lightshow that is cast by Archmage Vargoth DOES increase the increment on the card by one. Why is that? The card specifically says 'YOU'VE cast this game'. Could it be that 'basic mechanics' doesn't really mean anything? Because Vargoth either makes you (the player) cast it twice and doesn't cast it himself, in which case Vexallus should also work like the previous poster said, OR he casts it himself in which case the increment shouldn't go up. Either way you put it, something is wonky.
OP's question was fair for someone who didn't want to waste a loss on experimenting with funny mechanics encouraged by funny wording in a card game that's famous for being consistently inconsistent.
Your Nagaling doesn't have mana crystals to overload. If it casts a spell that says "draw a card" then you draw the card because it can't draw cards. if it says "gain armor" then you gain armor because minions can't gain armor. If it casts shield bash then it deals damage equal to your hero's armor for the same reason. "Shuffle into your deck" will shuffle into your hero's deck because the nagaling doesn't have a deck. Etc.
The Vargoth thing sounds inconsistent, though. I don't see why it should work like that.
In general I've found Hearthstone to be more misunderstood than inconsistent.