It's all quite simple really, you currently have XP installed and you want to add vista as a second OS that you can easily choose which one on boot.
1. decide whether you want to use a single drive or two separate drives.
2. once you decide then this will make your next move most important, if you are going to use the same drive and there are no other partitions on it or the other partition is quite large and hosts your back up files then you will need to create a partition using Gparted to create a partition without disturbing your current files.
3. If you are using 2 separate hard drives then you are ready to go (be sure to configure your drives in your BIOS for the boot order you need.. eg, drive 0 would be XP and drive 1 would be for vista.
once you have that, then also be sure all your hardware is Vista compliant, meaning that your hardware supports Vista. now you're ready to pop in your Vista dvd and install.
Note: be sure to back up all your important files before installing a new OS just incase Murphy's law intervenes