The safest vault a dwarf can build:
A glass tower
A tower spanning from the bottommost Z-level to the highest. Each story would have all sides except one corner covered in glass windows (the corner would be a wall supporting the above level). Inside the tower there would be only one tile room per story and that'd contain a cage with a goblin. The treasure would be on the second-highest level.
Pull a lever to release the caged goblins once you've built the tower.
After that, no dwarf could get to the treasure. Civilians would be too terrified of the goblins looking through the windows to go near it and military dwarves couldn't attack the goblins or destroy the windows.
I only came up with only three impossibly difficult ways to get the treasure:
1. Waiting until the goblins die of old age. Which they don't, because they have no maxage. But if one used other monsters to guard the tower, they could die of old age.
2. Rigging an accident to cause one of your dwarves to lose both his eyes and then keeping him alive. He wouldn't see the goblins inside the tower and could tear down the windows and build bridges around the tower.
3. Caging a really tough flying [BUILDINGDESTROYER] and then releasing it near the tower. But then you'd have another problem: a flying buildingdestroyer that just killed some 30 goblins.
But there's no fun in a vault like that.
Send a dwarf up to the second highest level as a soldier, then undraft him. Orchestrate a cave in to kill the goblins, or else build a drawbridge, drop the artifact on it, then deconstruct it to drop the item to the ground. Of course this assumes that the level with the artifact has walls to deconstruct for building materials and no goblin to scare the dwarf.
Alternatively, if the tower was not built up to the sky, a cave in could be arranged above the tower to drop the floor through the goblins.
Properly constructed though I can see this being impossible to crack.