Q2) If you try to install Windows 8 from within Vista, then you would lose Vista. You could use the tutorial below for how to setup a dual boot with Windows 8 and Vista when Vista is installed first. It's the same steps, but just substitute Vista for Windows 7 in the tutorial.
Windows 8 Developer Preview is an alpha version and just for getting a look at Win8, it's not recommended to use as your only OS. It's not polished, and buggy, some parts of the OS are not yet included.