@DvT: You could try simply duplicating the entire vault, renaming both vault folders and just deleting unneeded files within each vault. Both the local and global graph view should be quite handy in this regard.
I hope it works out. I find that this process of separating and combining vaults is terrifying even if you have things backed up. There is a deep fear of overwriting or disconnecting the system that is hard to shake. Yet, I regularly start new vaults saying this will be the last time. So far, it never was.
Good luck.
Thanks.