The Leather Vest of Flameproof is special in that while it doesn't give the wearer great physical protection, it makes them completely immune to any kind of heat based damage. Meaning that fireballs lose their danger, Fire Elementals are helpless, and it is even possible to walk on boiling lava without any damage!
The vest was once in possession of Silanus, a knight of the Guardian. However, he lost it on the third level of the Tomb of Praecor Loth. When finding him in Ultima Underworld II, he invited the Avatar to retrieve his lost gear. The Avatar found the vest not too far away from the ghost in the south-eastern quadrant of the level near a ladder. The vest proved a great help in defeating the Guardian.