VaultEvent

Each key used increases your chances of finding the main prize by 2.77%. You reach 50% chance at 19 keys.

Best case, you can expect to use 20 keys. worst case you will need to use all 36. 25-30 keys (69-83%) are normal find rates.

For vault quests do not level an artifact with more than one fodder piece at a time. It will count as a single level up regardless of how many pieces you use.