I needed to migrate vpopmail/domains multiple directories stored on 2 servers to a single vpopmail install.
Merging the two vpopmails user Maildir/ stored accounts actually actually was quite simple, though it needed a bit of tweaks.
Here is how I migrated the two vpopmail installations to reside on the one vpopmail mail storage.
1. I used tar to archive the two vpopmail installations on the two different
First I logged in with root over ssh on the first node, e.g.:
qmail-server1:~# cd /home/vpopmail/
qmail-server1:/home/vpopmail# ionice -c 3 tar -czvf vpopmail1_$(date +%Y%m%d).tar.gz domains/
...
Then logged in on the second machine:
qmail-server2:~# cd /home/vpopmail
qmail-server2:/home/vpopmail# ionice -c 3 tar -czvf vpopmail2_$(date +%Y%m%d).tar.gz domains/
I used ionice -c 3 which instructs the tar archive of vpopmail directory to be created using idle (sparing the hard disk) from I/O overheads.
Sometimes not using ionice especially the case where hundreds of mail domains exist in domains/ could overload the server and stop the qmail and even sometimes ssh from properly responding for a very long periods as well as delaying the work of the mail server and failure to accept some user connections to pop3 or smtp protocols.I found out about the exisnte of ionice command just recently, reading some blog online. The command is absolute "must use", always when have to archive directories with tens of thousands of files and subdirectories. It's also nice in general, also on some copy or move operations on GNU / Linux, ionice is truly great.
Anyways after a while I had my both archives on both servers existing, so I used sftp (one can use scp as well) to transfer the archives to the newly configured qmail + vpopmail installation.
2. Use sftp or scp to copy the archives to the new configured vpopmail server
Again on both servers I had to execute, sftp I prefer sftp as I love being interactive on the shell 
qmail-server1:/home/vpopmail# sftp root@my-vpopmail-server-host.com
Password:
Connected to my-vpopmail-server-host.com.
sftp> cd /home/vpopmail
sftp> put vpopmail1_$(date +%Y%m%d).tar.gz
...
sftp> exit
Once again the same commands has to be issued on the second vpomail server, where the second domains/ mail accounts archive was just made:
qmail-server2:/home/vpopmail# sftp root@my-vpopmail-server-host.com
Password:
Connected to my-vpopmail-server-host.com.
sftp> cd /home/vpopmail
sftp> put vpopmail2_$(date +%Y%m%d).tar.gz
...
sftp> exit
Now as both of the archives are uploaded, next step is to login to the root@my-vpopmail-server-host.com server, where the two archives were just uploaded:
qmail-server1:/var/domains# ssh root@my-vpopmail-server-host.com
my-vpopmal-server-host:~# cd /home/vpopmailmy-vpopmail-server-host:/home/vpopmail# tar -zxvf vpopmail1_$(date +%Y%m%d).tar.gz domains/
...
Next on, the second vpopmail2_$(date +%Y%m%d).tar.gz is untarred:
3. Unarchive the uploaded vpopmail*.tar.gz archives
my-vpopmail-server-host:/home/vpopmail# tar -zxvf vpopmail2_$(date +%Y%m%d).tar.gz domains/
...
Now, if the two archives are properly tarred and no errors are returned (that was my case thx God).
4. Use a shell script to generate a script, to later change all user emails passwords on the my-vpopmail-server-host.com
The next thing which has to be done is that all email accounts with passwords are recreated with vpopmail's /home/vpopmail/bin/vpasswd command. Even though the domains are existing with the respective vpasswd and vpasswd.cdb in each mailbox on the new server my-vpopmail-server-host still the mailboxes were not visible by vpopmail. To make all the mailboxes with the correct passwords be responding on the new vpopmail connections via IMAP and POP3 mail fetch protocols, I wrote a small script which does change the passwords of all mailboxes just transferred from the two servers to my-vpopmail-server-host
The script is very simple and actually is not too automated, but at least it works. I've called my script to dump all the user passwords for all the vpopmail filesystem existing mail domains dump_vpopmail_mail_passwords.sh
To use the script its necessery that the script is downloaded on both the vpopmail mail servers from which domains/ directory is migrated, e.g.:
qmail-server1:/home/vpopmail# wget http://www.pc-freak.net/bshscr/dump_vpopmail_mail_passwords.sh
...
qmail-server1:/home/vpopmail# sh dump_vpopmail_mail_passwords.sh >> change_mail_account_pwds.sh
Same procedure goes on the second vpopmail server qmail-server2:
qmail-server2:/home/vpopmail# wget http://www.pc-freak.net/bshscr/dump_vpopmail_mail_passwords.sh
...
qmail-server2:/home/vpopmail# sh dump_vpopmail_mail_passwords.sh >> change_mail_account_pwds.sh
5. Upload the change_mail_accounts_pwds.sh shell script created on the two qmail mail servers to dump_vpopmail_mail_passwords.sh
Again I used sftp to upload the two change_mail_account_pwds.sh bash scripts:
qmail-server1:/home/vpopmail# sftp root@my-vpopmail-server-host.com
Password:
Connected to my-vpopmail-server-host.com.
sftp> put change_mail_account_pwds.sh change_mail_account_pwds1.sh
...
sftp> exit
sftp>
qmail-serve2:/home/vpopmail# sftp root@my-vpopmail-server-host.com
Password:
Connected to my-vpopmail-server-host.com.
sftp> put change_mail_account_pwds.sh change_mail_account_pwds2.sh
...
sftp> exit
6. Execute change_mail_account_pwds1.sh and change_mail_account_pwds2.sh on my-vpopmail-server-host.com
On the 3rd server where the two vpopmail domains are migrated my-vpopmail-server-host.com , from /home/vpopmail invoke the above two scripts:
root@my-vpopmail-server-host.com:/home/vpopmail# sh change_mail_account_pwds1.sh
root@my-vpopmail-server-host.com:/home/vpopmail# sh change_mail_account_pwds2.sh
The two scripts will contain list with all the two migrated vpopmail user mail addresses in a format similar to:
vpasswd mail_account@mail-domain.com password1
vpasswd mail_account1@mail-domain.com some_password
vpasswd mail_accountX@mail-domain.com other_password
etc..
After following this steps, all the mail accounts (or most of them ) should be merged from the two servers on the new configured vpopmail on my-vpopmail-server-host.com host.
The up-described procedure could be literally followed with migratingm any number of vpopmail /home/vpopmail/domains user email accounts.
If it has to be mass deployed based, its coparatively easy even to write an automated script to do the various aforementioned steps without any user interaction.
I've seen also some other explanations on how to migrate qmail.jsm1.net , but honestly was too lazy to read them, so I just came up with the ways described here.
One important note to make here is always to make backups, this steps worked for me but I can't guarantee this migration tutorial will work at all. I hope this helps somebody out there. Cheers
