For those that don't see it yet: "Since PHP 4.0.4, libbcmath is bundled with PHP." also mean that it wasn't bundled by default for older PHP versions.
hmmmm, funnily enough both users are running php5 and the script works when run standalone it's only when run via vBulletin that their was an issue. however i'm checking on this. i really hope it's just a server configuration issue.