Problem 1: Troll guarding vault.
Solution: Fire hurts trolls. Burn troll with fire. Troll dies.
Note: Burnt troll leads to troll fat. Bring bowls to catch melted troll. Resell to Mages Guild.
Problem 2: Guards guarding vault.
Solution: Fire distracts guards. Mages Guild makes kindlepitch. Set fires in Mages Guild. Guards leave to put out fire.
Note: Burnt Mages Guild may prevent selling of troll fat. Find alternate buyer?
Problem 3: Vault has exceptional magical protection.
Solution: Find mage who devised security for vault. Threaten to burn mage OR burn mage, just a little. Slightly cooked mage reveals secrets.
Note: Troll fat poultice soothes burns. Offer to sell troll fat to slightly cooked mage?