Because Microsoft is no longer providing security updates for Vista, you are opening yourself up to malware if you choose to use the PC online.
Not that it's the only choice, but McAfee is the best and most comprehensive antivirus I know of.
If you're not into Win10, I'd think you'd be happier with Win7 than Vista. I Googled Win7 Pro support from Microsoft, but couldn't screengrab. "Don't worry: Microsoft won't end security updates for your Windows 7 PC until Jan. 14, 2020. For those of you using a Windows 7 machine, Microsoft is ending mainstream support for the operating system today. Don't panic, though. Your computer will still work and receive security updates. Jan 13, 2015"