I would install XP first. But you idea of partitions is off. You have to setup the partitions first before you install.
Vista XP Dual Boot - Understand the basics - The NeoSmart Forums
Read thru that. It will give you more insite.
As for your Hibernation. You got the wrong idea on that as well. You can only use 1 OS at a time. There is no way, unless you use Virtual Machines, to control 1 OS while in another. Dual booting you will either go to XP and use XP or log into Vista and use Vista. There is no hibernating XP and then using Vista. you will have to restart and choose Vista from the boot menu.