Windows 8 is not available anymore. You may be able to upgrade to Windows 10 as noted by Boom Ski. But your vista computer is quite old, it would be easier to buy a new one with Windows 10 pre-installed.
Yes you can
Of course, that won't be a problem
You should be able to. But you would have to do a clean installation instead of an upgrade. Also find out if you need to get the 32-bit version or the 64-bit version. There is also two different versions of Windows 10 you'll probably need the Home version is the pro is probably going to be a little bit more money and you won't use all the capabilities of it. And I think it costs about $120 not including tax. That should be with the CD version. For another 10 or 20 bucks more you can get a USB version of the operating system.
Thanks! Your answer is awaiting moderation.