Ok, first off, everyone thinks that Eragon will get his new weapon in some misterious way such as the famous smith making it for him. Just look back in book one, it says "When you need a weapon, look under the roots of the Menoan Tree..." There. That explains one part of this.

Next, I think the Vault of Souls is either in Heldigrind or right next to it. For all of you who have a book, look at the map CP drew. It has some small hills next to Heldigrind and something is on top/in the middle of them. I think that is either Brom's grave for in Book One it said that they were right in some hills when he died and he was buried on top, or that is they vault of souls. I can't tell.