Technically speaking Windows 3.0 was not an OS.
DOS 4.0, DOS 5.0 were the Operating systems. Windows 3.0 and 3.1 were applications that overlayed the OS....i.e, you could delete the Windows directory from the command line using DOS.
With the advent of Windows 95....Windows WAS the OS.
Nope, not even Win95 or Win98 or WinME were independent operating systems. They were all still just GUI applications over DOS. You could delete any of those Windows directories from the DOS command line (not from a DOS box within the GUI, of course).
In 95/98/ME, when DOS booted, it ran through AUTOEXEC.BAT, and you could set a line in MSDOS.SYS that would prohibit the automatic transfer to Windows at the end of AUTOEXEC.BAT. It would boot to DOS, and stop at the command line. You could type "WIN" and it would launch Windows. With WinME they made that harder to do, but WinME was still running on DOS.
With the advent of NT -- NT 3.1, 4, 5 (2000), 5.1 (XP), and 6 (Vista) -- Windows was the OS.
Yes, "Vista" is in fact merely "NT6". That designator is all over the Vista codebase -- the old 13-year old NT codebase. With some window-dressing, yes; but it's still NT.