A topic of an useful, healthy, friendly site should be interpreted as wide as possible, and should always contain what is the "common sense".
It is because new people won't know all of the relevant meta posts about the site topic, years retroactively. In most cases, they don't even know that a meta site exists.
And closing - destructing their content isn't a happy first experience. With their loss, the site loses all of the content as well, what they could have constructed later.
The content of the site should be filtered by quality, and not by an irrationally narrowed topic.
It is especially bad thing to use rules retroactively, it is the worst of all worst. Going years back on the site and deleting hundreds of questions on newly invented rules, it is simply... sulphuric, on my opinion. Instead of it, if such an intent comes forth, first it should be discussed, what is the real reason.
It is highly unrealistic, that something were about professional system administration years ago, and now it isn't. If you now want to kill year-old content, then something is nearly surely not okay.
Second important thing, that for such cases there is a healthier solution: using a tag with "historical" or similar name, to differentiate the old, but once allowed content.
Specifically to Vagrant:
First, constructing environments by Vagrant is a border case between the development and the administration. It shouldn't be offtopic, especially not on SO and SF, it should be ontopic on both sites. I don't think that a little bit of overlap with the SO would be a problem or even harmful.
There is a lot of cases where professional software developers need to work in environments constructed by system administrators for them. The difference between our common skills and viewpoints, it is big and highly visible. And, honestly, it is not always a good experience if both of us need to maximize our work quality, but on different measurements. Having "vagrant" content on the SF would be a help to build a bridge between the worlds.
But, if the vagrant content should be exterminated, it should be done on such a way:
- already existing, but migratable Vagrant posts should be migrated to SO
- non-migratable Vagrant posts get a "historical" tag
- new questions about Vagrant would be also migrated to SO (with the exception if they are crap. Then they would be only closed)
On my opinion, this is what should be done, but unfortunately the well-known, SE-wide disdain of the mods from the question migrations probably avoids the migration part.
But destroying content retroactively, it is always bad and it could be avoided.