Dockers underlying components are quite usable for many solutions in other projects and platforms. Now, instead of having "Docker TM" components in their stack, OSS projects can connect the tools without seeming to support a specific commercial provider.
Presently there is a lot of confusion and OSS projects are hesitant to use Docker components because of the perception of a closed eco-system. Now the project has clearer commercial branding and a more appealing package set for OSS consumers.
Remember: many of the most important actors we need to collaborate in this space are direct commercial competitors to Docker Inc.
But those projects aren't true OSS in the way that something like Docker / Moby needs to be. Your Vagrant example is one of them; Vagrant is a useful tool but ultimately too narrowly-focused. Docker's biggest coup was seamlessly integrating orchestration tools with deployment tools -- and now they're separating the two (which is completely fine, because they need to be separate tools anyway to support complex use cases).
Docker has built a lot of valuable IP that they want to protect (branding, UI, API, etc.) That makes a lot of sense. It also makes sense that they want to open source a big chunk of the "glue" between everything because 1) it's generally useful to lots of people / projects and 2) it gets other companies to kick in dollars for maintenance.
You're not going to get non-customers to fund the maintenance of an open source project that is wholly run and owned by Docker. They know that, and hence they're creating a governance board. But once you do that, you lose full control of your brand (which is attached to the project), and thus you have to divorce the two.
Docker is still going to drive the development of Moby. But over time, other companies will enter the fold to build other complex system orchestration projects on top of Moby.
Not the first time this is done either.
If they had renamed the commercial stuff to Moby they'd have to start from scratch, now there are thousands of articles and links that say 'go get docker' intending to point you to the open source stuff, instead directing you to the commercial stuff.
A very effective bait-and-switch.
That happens ALL the time. Except they (managers) don't read tutorials, they read trade magazines saying that X is the new hotness.
So off they go and buy X Enterprise Edition.
This happens in the corporate world, not everywhere, but that's where a company gets the big contracts.
Not picking on Docker here, it's a great product and good luck to them; just a commentary on corporate fashion driven IT culture.
That's not Docker. That's Moby.
By splitting up the open-source Docker CE codebase into smaller projects, each smaller project is more approachable to outside potential contributors. If Docker can succeed in increasing the number of contributors to the Moby projects, compared to the Docker CE project, then they'll have succeeded in increasing the relevance of their product in their target market, which translates to more customers, sales, and profits in the long-term.
Of course they run the business risk of a fork taking off, but in the real world, forks are arms races. They're easily won against dead or severely mismanaged projects, but against a project with a well-funded corporate sponsor? Few, if any, will have the resources to put into making a better fork than the original.
And like they said, for the vast majority of their customers, nothing is changing. Their customers bought a Docker product yesterday, and they're buying a Docker product tomorrow. That branding isn't changing.