We know that house-elves can apparate into Hogwarts, where wand-carriers can’t. Could they do the same with a high security vault at Gringotts?
This is not described in canon, but I would guess that the answer is no.
Two reasons for this:
Gringotts’s goblins will not overlook house-elf magic like humans do.
This is spelt out most explicitly when Kreacher is explaining how he explained the cave:
“Elf magic isn’t like wizard’s magic, is it?” said Ron. “I mean, they can Apparate and Disapparate in and out of Hogwarts when we can’t.”
There was silence as Harry digested this. How could Voldemort have made such a mistake? But even as he thought this, Hermione spoke, and her voice was icy.
“Of course, Voldemort would have considered the ways of house-elves far beneath his notice, just like all the purebloods who treat them like animals…. It would never have occurred to him that they might have magic that he didn’t.”
— Deathly Hallows, chapter 10 (Kreacher’s Tale)
The protections around Hogwarts were probably cast by the four Founders, who were all humans. It’s unlikely they gave much thought to house-elves when designing the defense, whether their magic or the idea that a house-elf apparating into Hogwarts would pose a credible threat to the school.
On the other hand, the goblins
are intimately familiar withhave more respect for non-human magic, and would be actively looking for loopholes like this. We don’t know what goblin feelings towards house-elves are, but they’re not stupid enough to dismiss them.
I would also highlight that goblins do not trust humans, and will be actively looking for loopholes that humans exploit. Pulling a trick with house-elf magic like Regulus does above is the sort of thing the goblins would expect, and guard against.
Hogwarts security does not need to be as rigorous as Gringotts.
The Hogwarts security seems relatively pervious. There are multiple perimeter breaches described in the books:
- Charlie and his friends fly in and out to collect a baby dragon
- Harry and Ron breach the castle grounds in a flying car
- Sirius slips past a pack of Dementors in the form of a dog
- Fred and George leave the school on brooms
and I’m sure there are others I’ve forgotten.
In general, the Gringotts security seems a bit more robust. (Quirrell’s break-in and the trio’s escape on a dragon aside.) I’m sure being an Animagus or using brooms isn’t sufficient means to break into Gringotts.
The Hogwarts protections are not designed to be absolute. The Gringotts protections are, and that surely includes all forms of apparition.
House Elves can Apparate into Hogwarts because it's a necessary part of their function, allowing them to clean and go about their duties without being seen. It's not so much a bug as it is a feature.
The only other instance we have of House Elves Apparating in places wizards can't is Kreacher in 12 Grimmauld Place (which again, is necessary for his duties) and Kreacher again in the Horcrux cave. The second one I think can be chalked up to an oversight on Voldemort's part, who is shown to constantly underestimate and overlook magic he considers beneath him.
But there is no legitimate reason for House-Elves to pop in and out of high-security vaults. If there was such a massive flaw in Gringotts security, I imagine that the Trio and Griphook would have stumbled upon it in their month or so of planning the robbery (Especially given Dobby's role in escaping Malfoy Manor). After all, if there was any House Elf capable of entering Bellatrix's vault, surely it would have been Harry's elf Kreacher.
If Gringotts has similar security as Hogwarts then yes as wizards don't every account for the magic of magical creatures. Gringotts on the other hand is run by goblins. Goblins make items that are resistant to all magic. Goblins also would not necessarily view house elves as ignorable creatures, since Goblins themselves are treated that way by humans. Also since house elves do whatever you tell them to, I'm sure after the hundreds of years gringotts has been around someone would have tried it. So canonically we can't know, if its protections against just wizards then yes they can, if goblins thought of it (they are quite crafty) then no.