You can buy another HD and install the XP on it if you get the XP license. But I do not think you want to open the HD cage to swap it out. Best way is to set up dual boot. But it works best if you have XP, then install Vista later. Not sure what if Vista would allow you to install XP on it. You may have to get a boot software to take care of it. It is not worth the trouble with a laptop unless you have hours to kill. Keep the Vista.
You can, but depending on the laptop it could be a royal PITA. First question I'd ask is why do you think you have to run both OS's? If one of them is only for occasional use you're better off using it via a VMware virtual machine. If you need to run both frequently you could create a dual boot machine, but you're going to need a fairly large HD because the only way you can do it is by hiding the unused OS. Aside from Windows having an absolute shit-fit if it finds another WinOS boot drive, you can't have two C:\ drives visible on one HD and you simply don't want to try booting and using it off of D:\ To do it you'd need to load Linux and configure Grub to swap and hide each OS partition within the menu.cfg file. The details are documented pretty well in several places on the 'Net.