Léonie's advise is basically correct (except for that weird reference to the iBook! I think she meant the iMac there )
One thing I would suggest is that after you merge the libraries I would just go ahead and create a new vault from the merged libraries. As a vault is just a snapshot of the current state of the library there would be no advantage to updating the current vault and with the amount of changes the merge will probably cause to the library an update is going to most likely take as long as creating a new vault would.
(except for that weird reference to the iBook! I think she meant the iMac there )
Oh, no! At least you got a good laugh out of it. I had to leave in a hurry this morning and never had time to check what the spelling correction did to my post. I am quite accustomed to the spelling correction turning each mention of "Apple" into "Aperture", which can be somewhat embarrassing in the GarageBand forum, but downgrading a MacBook Pro or iMac to iBook is new to me.