assume you are running example.com with qmailtoaster.

1. install latest iredmail which significantly simplify the whole process of installation and configuration.
2. dump and import vpopmail  into new db.
3. copy  /home/vpopmail/domains/* to /var/vmail1
4. chown -R vmail:vmail /var/vmail1
5. run following to move all the accounts into postfix vmail.mailbox table.
6. done

INSERT INTO vmail.mailbox ( username      , password                               , name      ,language  , storagebasedirectory   , storagenode   , maildir                , quota , domain           , transport , department , rank     , employeeid , isadmin , isglobaladmin , enablesmtp , enablesmtpsecured , enablepop3 , enablepop3secured , enableimap , enableimapsecured , enabledeliver , enablelda , enablemanagesieve , enablemanagesievesecured , enablesieve , enablesievesecured , enableinternal , enabledoveadm , `enablelib-storage` , `enableindexer-worker` , enablelmtp , enabledsync , enablesogo  , lastlogindate       , lastloginipv4 , lastloginprotocol , settings                  , passwordlastchange  , created , modified, expired               , active , local_part )
SELECT                    concat(r.pw_name,'@example.com') , r.pw_passwd , r.pw_gecos, 'en_US'  , '/var/vmail'           , 'vmail1'      , substring(r.pw_dir,24) ,     0 ,'example.com' , ''        , ''         , 'normal' , ''       ,       0 ,             0 ,          1 ,                 1 ,          1 ,                 1 ,          1 ,                 1 ,             1 ,         1 ,                 1 ,                        1 ,           1 ,                  1 ,              1 ,             1 ,                 1 ,                    1 ,          1 ,           1 ,          1  , '1970-01-01 01:01:01' ,           0 , ''                , '' , '1970-01-01 01:01:01' , now()   , now()   , '9999-12-31 00:00:00' ,      1 , r.pw_name
FROM vpopmail.example_com r;