I'm not sure this is possible without losing your recovery volume.
Its worth trying booting into the original copy of Vista and then using Computer to navigate to the partition where the unwanted copy of Vista is then deleting all the operating system files (be careful not to delete the recovery files!). There may be a few which cannot be deleted but not many. That said, it will be a one way process and if it doesn't work (i.e. speed the laptop back up again) then you'll most liekly end up reformatting the partition and yopu'll lose your recovery info regardless.
Before you do anything, make a system restore disk!!!!!!