Of course you can, but you have to install the older OS (2000) first,
then VISTA. Also, you must either partition your Hard drive into
two sections, or use 2 Hard Drives.
Since you already have Vista, you will have to load 2000 on another
hard drive while Vista's HD is disconnected. Then get some third party
boot loader, or use your BIOS to select the boot media. Last choice is
to axe Vista and start from scratch, installing 2000 first, then
Vista.