If you go to Dell's website and search for a driver, see Drivers & Downloads
, there are drivers for Vista 32bit *and* Vista x64 that you can pick from - they show the NVS 110M and 120M as both being suitable.
Therefore, Searching on nVidia's site for drivers I see that NVS for Notebooks only goes down to the 120M series - but that links to a Vista driver dated 18 December 2008 (about 21 days ago) that is listed as:
Quadro NVS Release 179 for Notebooks BETA Version
: 179.28 Release Date
: December 18, 2008 Operating System
: Windows XP, Windows XP Media Center Edition Language
: U.S. English File Size
: 126 MB
This is a reference driver that can be installed on supported NVIDIA Quadro NVS notebook GPUs. However, please note that your notebook original equipment manufacturer (OEM) provides certified drivers for your specific notebook on their website. NVIDIA recommends that you check with your notebook OEM about recommended software updates for your notebook. OEMs may not provide technical support for issues that arise from the use of this driver.
However, since they say to use the OEM's drivers, I highly recommend you go to the Dell page and search for your driver using your machine type - Laptop --> Latitude --> D820