Hello Rozsomak, and welcome to Seven Forums.
Since you will be using a separate hard drive, then you would use step 1 in the tutorial. When done, your computer will start Windows 7 by default since it was the last OS installed. You can change the default OS to have startup
anytime you like though.
When you are in Vista, Vista will show as the C: drive and Windows 7 as for example E:. When you are in Windows 7, Windows 7 will show as the C: drive and Vista as for example E:. The OS that you startup to will always show as the C: drive by default and cannot be changed.
You can access the files from each OS from within either OS, but you can only run programs that are installed on the current OS.
Hope this helps,