I messed up by deleting a plugin and now when uploading a similar one its giving errors. Is there a way of restoring just the plugins using a backup I have?
In theory you could restore the backup to a different database name, change your config.php to point to the restored database, turn on debug mode, then go to the Product Manager and export that product to an xml file. Then switch back to the current database and import it. (You'd want to close your forum while doing this, I think).
It could also be done using a test site, or maybe just by finding the right rows in the right tables in the old database and transfering them to the current db, but of course those would be a little harder to do.
Well, that's the easiest way I can think of. But that method wouldn't restore any mod settings you may have changed. Anyway, maybe someone else will have an idea.
From that error, it looks like maybe you've installed a vb4 mod, in case that helps at all.