You can dual boot Vista and XP.
Vista will be your default, I don't think you can change that change that.
I know it's easier to install XP and then Vista.
You will need to partition your HDD in order to dual boot, I don't think there is any way around that.
Being that you don't have any restore DVD's, you will have to find some software to partition without loosing any data.
Is your HDD large enough to have two full partitions and OS's? You will have 2 copies of all of your software. You can share your personal files, but I am unsure if you can use the same folders for "My Documents" and such.
If you install XP after Vista you will need to repair the Vista boot loader (?) in order to boot Vista. I will see if I can find the web page I used after I screwed up my PC.
I would recommend looking to see if you can make your own restore DVD's before you do anything. I had an HP that had a program that let you burn a set ONCE!
I hope This helps and not confuse you more.