there was an update to reward vault in the past adding the missing items to it in the PvP cluster (before it died) I thought maybe they could add this item to reward vault (...)
I saw Dani making changes in the event server and it didn't even require a server restart to add or remove items from reward vault.
You're absolutely right, items can be added to the vault via vault's admin UI on a live server, but the item still must exist! And since it's not in game files, it would have to be added with a mod - either this potion mod, or with GD rewards mod if Gunnsen ever finds time and will to create such item (create, not copy-paste, since the original mod isn't open source)