To answer your questions in the order you have posted them:
1- Yes you can dual boot Windows Vista (x64) and Windows 7 (x64)
2- You need to install old to new.. So Windows Vista would be first, followed by whichever build of Windows 7 your going to be using. (I'd suggest waiting for the Windows 7 RC, but of course it's up to you..)
3- My suggestion would be to use EasyBCD to set one OS as the default OS... This app allows you to customize the boot manager however you want. It's available here: Download EasyBCD 1.7.2 - NeoSmart Technologies
(It's very easy to use)
4- You shouldn't run into any problems with dual-booting as long as you install in the right order and make sure you install the appropriate drivers for each OS... I have 5 PC's at home, 4 of which are dual boot and 1 is quad boot and I've had no significant problems at all with any of them so you should be fine.
5- Yes you are correct, if down the road you decide you want to remove Windows 7 than yes all you'd have to do is format the partition with Win 7 than your good to go.. (You may have to reconfigure your boot manager afterwards but it's not problem to do this using EasyBCD).
To add to this discussion I'd just like to ask how you plan to setup your OS partitions.. You said you have 5 HDD's in total right? If so, then I'd suggest creating a partition on 1 HDD of about 25-35GB's for Vista than create another partition on another HDD of about 25-35GB's for Windows 7. Then the rest you can devide up to your liking of course..
I guess my point here is to put each OS on it's own HDD... I have all my PC's setup that way and like I said I've had no significant problems at all... You can put each OS on the same HDD (each on it's own partition of course) but I find sometimes that can lead to problems or poor performance.. Just a suggestion though..
I wish you luck and hope to hear from you when your all up and running. If you need anymore help at all don't be scared to ask. We're all glad to help in any way possible here at Windows7forums..
Also if you need help with EasyBCD at all just PM me and I'll help ya out however I can...