Can you add additional disks to your server? I would highly recommend moving tempdb to its own drive, and preferably a large one just in case.

If this has been working fine until just recently, you may have reached a point where your data has exceeded what the system was originally designed to work with and definately needs some tuning by the vendor.

Adding disk is only a band aid here but is the first step I would take while getting the vendor to look at their code and hopefully modify it to work more effeciently.