The Inspiron 9400/E1705 lid switch is supported under Vista natively, aka it operates separately from the NVidia driver. I would assume its the same switch from the 9300. Look under your device manager at the system devices. There should be one called ACPI Lid in there. If there isn't, something isn't right. If it is there, try removing the device from there and reboot. See if that helps.
Aside from that, your lid switch should work under Vista from your very first boot - even with the Microsoft WDM driver installed. I have used many drivers beyond the originals and none of them interfere with the lid operation...
Might suck, but try another Vista reinstall and check the lid function before you install aftermarket NVidia drivers.