The steps to migrate your vtiger CRM 6.x to coreBOS 5.x are:
get a copy of coreBOS from github
recover a dump of your vtiger CRM 6.x database
copy your config.inc.php from VT6 and modify the database settings to point to your copy
at this step your coreBOS install is pointing at the vtiger CRM 6.0 and does not work
copy the migration script to the root of the coreBOS install. The script is in the build directory
build/migrate_from_vt6.php
execute the script
http://your_server/your_corebos/migrate_from_vt6.php
http://your_server/your_corebos/migrate_from_vt6.php?force=1
copy all your documents from the vtiger CRM install: directly copy all the files in the storage directory from one to the other
copy logos and the like which are in the test/logo directory
enter coreBOS and activate modules in module manager if necessary
apply coreBOS Updater changesets