Sometimes the vTiger installation cannot be completed successfully because the database is not populated with the needed data. To avoid this issue, prior to installing vTiger, you can make a small modification to the install.php file. The line you should modify is:

ini_set("zend.ze1_compatibility_mode", "1");

Change it to:

ini_set("zend.ze1_compatibility_mode", "0");

and the installation should be completed successfully :)

You can modify the install.php file using the File Manager in your cPanel.