Hello Machomer,
First off, from the sounds of it, you are doing the right thing in backing up your files, doing a fresh install of Leopard, then bringing all of your files into that new installation.
Using the option to "Archive and Install" over the former OS and just keep all your personal files on the computer has been giving many people headaches (including myself). So, I would Definitely suggest doing an absolutely fresh install to just bypass those weird issues. Then, when the install is done, just choose the option to bring your files from another Mac onto that one (from your backup disk to the internal), and you'll be all set there.
Now, to back up your Mac files, I'd use something like Carbon Copy Cloner for more than one reason (link at bottom of post). First reason: it's free. Secondly, it'll clone your various folders instead of just copying them, so you're sure not to loose hidden files. (Note: It'll ask you for your administrator password before it will do any actual backup. This is because they want to protect your files from whomever might want to steal your data. It's perfectly safe, so don't worry about that).
About your VMWared Vista bit: VMWare uses a file to save your Windows install in, so you don't need to "backup" that bit. Unless you've moved it, this fake partition will be in your Documents folder in a folder named "Virtual Machines." If you just use CCC to backup your Documents folder, the VMWare bit will be automatically brought onto your external drive. And, yes, you will need to reinstall it most likely, because I'm pretty sure VMWare runs different versions/builds for Leopard than earlier versions (hardware/OS/path differences). You will not, however, need to buy a new key. If you bought it, it's yours. Whether you upgrade OS's or switch machines, it's yours. Just make sure you only have it installed on one, and legally, you're fine. If you do have any issues with that, call their tech support and they'll straighten things out for you.
BootCamp will create a partition (if you choose to do so), but your VMWare OS will not run there. You would need to install Vista straight, and then boot into it instead of Leopard (and have to go through probably hours and hours with Windows tech support to get your key re-activated, because Windows has a "one and done" policy when it comes to their OS installations). So, I'd just stick with VMWare within your Mac partition. Plus, you then wouldn't be giving up space to Vista that you aren't really going to use, and will not be able to get back without re-backing up, wiping the disk, and reinstalling.
Also, with some versions of Vista (i.e. what "edition" and where you got it from), I've read there is an option to call Windows and get a "downgrade" key or something. Might be worth a shot, if you don't mind spending a "few" hours on it. I know I like XP a lot better from the little bit I've seen.
Hope that wasn't too long and actually helped. If you have any more questions, feel free to post and we'll help how we can.
-NewFroggy
Carbon Copy Cloner:
http://www.versiontracker.com/dyn/moreinfo/macosx/13260