no you cannot install on top, you have to clean install it.
to boot from disc just put in ur vista 64bit cd and when your booting youll see something saying something to this nature "Press Del To Enter Setup" it may be F12 as well .. hit Delete or F12 at that time to enter your Bios. find Boot Priority and set CDROM to first boot device or slot #1 exit and save. boot up and when it says "Press any key to boot from CDROM" hit a key and there you go. Make sure you change your boot device back to what it was after you install vista.. unless vista screws up your BOOTMGR like it is for most of us (in which case you don't want to change it back because you need the CD in to boot into vista)