If every OS was open source, you could run vista
and it'd be fine
It had a poor reputation. My vague recollection is that I experienced an irritating number of driver-related BSODs. To me, it now feels like a standard Microsoft experience of them pushing out a new OS then fixing it with service packs over a few years.
To answer OPs hypothetical thought-experiment:
In my view, running Windows Vista today would firstly have the problem that Microsoft has added lots of extra stuff (OS APIs, runtime libraries etc) and you would find that quite a lot of software won't work without stuff in newer versions of Windows.
I feel that if someone added enough extra stuff to Vista to run current software to Vista that it could run most current software then it would not really be Vista anymore.
Most of the Window XP source code has leaked. As far as I know, no pirates are patching, recompiling and releasing versions of it.
Patching Windows security holes is a lot of work. I think Vista was vulnerable when some NSA tools leaked in the year 2017 and it was possible to remotely seize control of Windows machines if port 445 was reachable and things like the wannacry worm caused global chaos.
I'm fairly sure that when Vista reached EOL it still had a lot of LPE vulnerabilities.