when searching a vault using no parameters and simply selecting "Search" I am given a certain (2499642 found)
http://localhost/enterprisevault/listvaults.asp will list all vaults with a "Total Items" column.
Which is more accurate, what do each consist of?
I don't know for certain but suspect the descripency is due to the SIngle Instance and Compression that EV applies to items. With the Journal you also have a fan out factor depending on the number of journal servers/mailboxes that EV is archiving from.
1 message sent to 5 people could be sent via 2 or 3 journal mailboxes, each with their own message ID so will be seen by EV as seperate messages. Tools like DA can later dedupe these.
The usage page is not the volume stored on disk (this will be smaller due to the SI and compression), or the raw data volume as this is what is sent or received before any SI or compression. This is the data that EV has processed including SI (but not compression)