* How much room should I allot for the virtual hard drive? I set up 12GB for it on the first attempt but after a standard VS2008 install (which kept failing) I would have been left with only 500 MB
I think you are going to want at least 40-80GB to install Vista and all your development tools. It's always better to have more than less. You could always mount additional virtual disks if you run out of room (I forget if VirtualBox can do this).
* How much base memory should I allot for the VM?
The base should be 2GB. However, if you turn off all the uneeded things you can get away with less. Luckily this is easily changed in a virtual machine.
* Any other tips/tricks/advice? I'm not completely settled on any part of the process (though I'm pretty sure I would like to try Vista)
If you are going to use Vista you should turn off all uneeded services, indexing, system restore. Turn off all effects as well. Don't use Aero (although don't think it gets activated in a virtual machine).
As others have said Vista isn't a very good OS in a virtual machine. I have tried it in Virtual PC and Virtual Box and neither run well. Virtual Box is faster for me but I still would not use Vista. I would use XP pro in a Virtual machine. But if Vista works well for you that's great.