It is not possible to download Windows Vista as Microsoft no longer offers the operating system. As of 2015, Windows 10 is the latest version of the operating system and is the only version that Microsoft sells.
The only way to acquire a copy of Windows Vista is to buy a physical copy. Unopened Windows Vista DVDs are available at Amazon.com. However, Microsoft is planning to drop all support for Windows Vista in 2017.
Windows 7, 8 and 8.1 users can download Windows 10 for free. The operating system automatically downloads the newest version and warns the user when it is ready to install.