Hi bekidur,
I do not quite understand what you are looking for, because the question jumps arround a bit. However this might answer some:
The Client Extension can use DCOM or HTTP to speak to the Enterprise Vault Server.
You can install the client well before you enable the mailbox for archiving. The client will only get activated once the mailbox has been enabled for archiving.
When you Enable the mailbox for archiving, Enterprise Vault writes a hidden message into the mailbox, the client reads the settings from there.
Hope this helps.
--wayne