Double Vault MK I (OG Double Vault) - Credit: aGamingGamer, AussieOtter
Back in this day, before the Spotbusters, or before Coconut Brah had pivoted to parkour videos, shields were able to vaulted on from the side when placed near an object, and would break upon the vault animations completion, not allowing a vault afterwards. This was able to be bypassed by having someone pick up the shield, and for a small time afterwards, the vault would still be on the shield. During this pick up animation, the shield would no longer be able to broken, allowing the vaulting player to perform a vault after vaulting the shield.
Double Vault MK II (Kiba Double Vault) - Credit: Banana, Goofy Balls
The simple method of patching the original way of double vaulting was to remove on-vaults to a shield, making them unvaultable from the sides when placed near an object. This was simply bypassed by using a kiba barrier to vault on top of the shield, since shields were not yet anti-landing zoned. The principle of the shield being unbreakable stayed the same, allowing the player to vault over a kiba onto the shield, while the latter was being picked up.
Double Vault MK III (Osa Double Vault/Attacker-Defender Double Vault) - Credit: Nam-Nam, Plo
The next double vault was not found until the early days of the Spotbusters Public server. Player collision between attackers and defender was found to very strange, and it turned out, a double vault, this time involving Osa, was possible. Whenever an Osa shield was picked up, and a defender would vault right before the prompt would go away, a double vault was able to performed. Additionally, another version of this same double vault could be performed by having Osa be on rappel with her shield out. A kiba could the be placed and vaulted over to land on the shield, and a double vault could be possible.