Ok here is what I would do.
1) Turn your site off.
2) Make a full backup, just incase something goes wrong.
3) Re-upload the files that came with your v4.2.2 zip.
4) Run the upgrade script.
5) Then get a copy of v4.2.1 Pl1 upload those files to your server.
6) Turn on debug mode by adding the following to your includes/config.php
Code:
$config['Misc']['debug'] = true;
7) Go to ACP --> Settings --> Options --> Version Info and Other Untouchables, and change ONLY the setting, vBulletin Version Number to 4.2.1
8) Turn off debug mode by removing the following to your includes/config.php
Code:
$config['Misc']['debug'] = true;
9) Remove the install directory.
10) Turn your site back on.
Now you will be upgraded to v4.2.1 and not have to worry about the 4.2.2 and PHP errors.