A few issues and Recommendations...

First:

You cant save single items... That you already know probably

second:

If you could make it so its like the custom kit plugin that would be great! kinda like:

/vault <save|Load|Delete>

By doing that that would make it nicer.
like in the database
[id] [steamid] [name] [contents] [lastmodified(timestamp)]

Third:

The MySQL is so unorganized. If i am to save a large amount of items, have it so its not as long of a line.
also. get rid of the server_id and 'dont save across servers' section in config because if i dont want it to save i would just change the database table name...
ex.

[Id] [Steam ID] [Inventory] [Timestamp]
2 steamid64 1:6, 72, 15:3, some day

and make it so it neatly sorts item amounts like this (if possible)

If you can only do one of these requests, please try to fix the issue and add the reccomendation with the vault names etc...