If your computer has hardware virtualization support (Intel's iVT or AMD's AMD-V) then it'll be close to native speed, but you need to be aware that you won't have hardware graphics acceleration, so no Vista aero or graphical games.
I think all Core 2 has iVT.
Regarding how to do it.. you already have KVM there, as Virtual Machine Manager. Create a new VM from within there, make an 8gb or whatever file to use as the disk, select non-paravirtualised (i.e. not Xen), but tick hardware acceleration and you'll have a system ready to install Vista onto. You have to map the virtual Cd drive to /dev/scd0 or whatever your cd drive might be.
With regard to just booting up your existing Vista installation within a VM, that's not going to happen unless you do something like image the drive with Acronis TrueImage and then restore that image to the VM with Acronis' optional Universal Restore feature. Might as well just not bother because it's a bit too involved and likely to go wrong.
I would think there are loads of "how to install Vista virtual machine on Hardy Heron" howtos out there!