I am not sure how Vista would treat this but I have done it in the past with Windows XP already installed and then I partitioned my hard drive and installed another OS.
A separate drive would be fine, as you can simply change the boot order in the BIOS when you want one OS or the other. XP/Vista dual-boots are quirky on a single drive and requires full CDs for both, but here's the basic gist if you want to give it a shot:
1) Install vista.
2) Create an extended partition in Vista with diskmgmt.msc
3) Boot XP CD and install to that copy of XP
4) At this time, XP will overwrite Vista's boot loader so Vista will be inaccessible.
5) Put your Vista CD in and perform a repair install on the Vista installation which will rewrite the Vista MBR, add XP to the boot list, and give the ability to dual boot.
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How about running XP on a separate hard drive in the computer or in a USB enclosure? Is that the same thing?
1) Install vista.
2) Create an extended partition in Vista with diskmgmt.msc
3) Boot XP CD and install to that copy of XP
4) At this time, XP will overwrite Vista's boot loader so Vista will be inaccessible.
5) Put your Vista CD in and perform a repair install on the Vista installation which will rewrite the Vista MBR, add XP to the boot list, and give the ability to dual boot.
YMMV significantly.