Vista x64 supports UEFI. No need for CMS mode. Just leave the settings their default, except turn secure boot OFF. Thats all you need to touch in your UEFI firmware.
What you need to do is this:
You need:
-A Windows Vista 64-bit installation DVD with a valid product key
[DEL]-A blank USB stick.
-Download
this (KYHI's Windows PE environment (boot disk), found in his signature)
-Download
WinRAR[/DEL]
Seeing as you already have your partitions set up, all you need to do is prepare the USB to make it bootable and extract KYHI's ISO to it.
[DEL]1. Open a Command Prompt window as an administrator. (Win+X, "Command Prompt,(Admin")
2. Run the following commands
diskpart
list disk
select disk [x] (x being the number of the disk that is your USB, the list disk command gives you the size of each disk in MB, you will tell which one is the USB by the size, my 8GB one is 7648mb.)
clean
create partition primary
select partition 1
format fs=fat32 quick
active
exit
Now close the CMD window and navigate to where you downloaded KYHI's boot disk
Right click ---> Extract files
Select "Removable disk" this will be your USB
CLick "OK" and it will extract the files to the USB[/DEL]
Now, put your Vista disk in the drive (keep the USB connected), [DEL]reboot and access the boot menu (generally if you press "Esc" on bootup it will give you the hotkey to access the boot selection)
Boot from the USB stick
It will boot into the PE environment.
[/DEL]
Instead of doing all that, just download WinNT setup from KYHI's post just below mine
Now open WinNT Setup 3.5, and open with it Vista's
install.wim found on the Vista disk\sources\install.wim
Check any modifications/customisations you wish, but make sure you check the "Find existing Windows and add to the boot entry" then click "Start"
WHen it is done it will tell you, then remove the vista disk, and restart your computer
You should be presented with 2 options at boot
"Windows Vista" and "Windows 8.1"
Select Vista to enter the System OOBE. (User account setup etc, and install drivers.)
No idea why you would want to dual boot an OS from 9 years ago that is also the worst OS MS ever made, but there you have it
The same tutorial can be used to dual boot Win7 if desired.