Hephaestus and Medusa had a son named Cacus. How was Hephaestus shielded from being turned to stone?
Hephaestus and Medusa had a son named Cacus.
Cacus was the son of Hephaestus and Medusa. Inheriting the properties of fire from his father and monstrousness from his mother, Cacus was a fire-breathing monster who was sometimes described as a three-headed giant. He lived on human flesh and decorated his cave with the bones and skulls of his victims.
How were Hephaestus and Cacus shielded from being turned to stone? Does being a God shield them?
Great question! I didn't know about this creature at all. I took the liberty of editing the question with a reference to this not-so-well-known creature. Please, feel free to rollback if you don't like this change.
Perhaps Medusa being Cacus mother can be traced to John Lemprière's "Bibliotheca Classica"?
I don't really consider that a particularly good source. Virgil says that Cacus was Half Human, in the Aeneid, Book VIII:
There was a cave here, receding to vast depths,
untouched by the sun’s rays, inhabited by the fell shape
of Cacus, the half-human, and the ground was always warm
with fresh blood, and the heads of men, insolently
nailed to the doors, hung there pallid with sad decay.
Vulcan was father to this monster: and, as he moved
his massive bulk, he belched out his dark fires.
Now at last time brought what we wished, the presence
and assistance of a god.
Additionally, I have been unable to find a single corroborating citation to your source about Medusa being Cacus's mother. Especially since, as a Gorgon, Medusa was a Daughter of Phorcys and his sister Ceto, ancient Cthonic deities, and was therefore not human.
Finally, your source says that Cacus inherited Monstrousness from his mother, but we know that other children of Hephaestus were also monsters, for example in Apollonius I:
 With him came Palaemonius, son of Olenian Lernus, of Lernus by repute, but his birth was from Hephaestus; and so he was crippled in his feet, but his bodily frame and his valour no one would dare to scorn.
Side note: Being a God did not shield one from Medusa. See: How did Athena keep from being turned into stone by her shield?
_Especially since, as a Gorgon, Medusa was a Daughter of Phorcys and his sister Ceto, ancient Cthonic deities_, does this mean she can't have children?
@Daft No, it means any children she would have had with Vulcan wouldn't be human (or half-human), as neither she nor Vulcan were human.
Could Virgil's description of Cacus be just that- a physical description? So that Cacus might still be the son of Medusa and Hephastus, but Virgil calls him half-human as a description of his body?
Sorry for referencing wiki, but I don't have better source. I've just remembered this during reading of this question.
In a late version of the Medusa myth, related by the Roman poet Ovid (Metamorphoses 4.770), Medusa was originally a ravishingly beautiful maiden, "the jealous aspiration of many suitors," but because Poseidon had raped her in Athena's temple, the enraged Athena transformed Medusa's beautiful hair to serpents and made her face so terrible to behold that the mere sight of it would turn onlookers to stone.
In Ovid's telling, Perseus describes Medusa's punishment by Minerva (Athena) as just and well earned.
So if I assume right, not in all her life was Medusa "able" to turn anybody into stone. If she had given born to Casus before her transformation, he didn't have to be "shielded".
None of the quotes relating to Cacus on Theoi.com mention who his mother was, only that his father was Hephaistos. Cacus was only known for trying to steal Hercules' cattle, and as a minor character he wasn't provided with a lot of backstory, which might explain why we know so little about him.