DP, before you go dual-boot, there's always
VirtualBox, which runs D2 just fine, if you
disable 3D acceleration.
(Added bonus of Suspending the VM mid-game, saves time loading the game

)
Oh, and have you tried the download versions of D2? If you register your key with blizz, you can download an installer from them (doing this w/ StarCraft right now...), maybe those work better... Compatibility Mode anyone?
Disclaimer: I do not have a Vista install, and as such am not able to validate any claims. But looks like I need to get one soon, to be able to help people better. Know Your Enemy, as I say...
