You just need to install OSx86 on new partition. It does not matter if it's on the same or different hard drive with the Vista.

I installed Vista and iPC OSx86 on same HD with 4 partitions (Vista OS, NTSF data, Leopard OS, MAC data). You may get error missing MBR when booting up to the Vista cause by OSx86 installation; just do a setup and repair of Vista to get it back? After boot up with Vista, download and install free software EasyBCD and configure the dual boot.

You can add an entry by select corresponding OS and HD partition as show in image



EasyBCD is NeoSmart Technologies' multiple award-winning answer to tweaking the new Windows Vista bootloader. With EasyBCD, almost anything is possible. Setting up and configuring Windows boot entries is simple, and there is no easier way to quickly boot right into Linux, Mac OS X, or BSD straight from the Windows Vista bootloader - on the fly, no expert knowledge needed!