tl;dr
Can you specify which things in the diagram you mean by “vendor-specific namespaces”?
Most important is that in any ActivityPub or LD anything can have multiple type - the targeted app will choose the best (most narrow) type …
So, for example {"type": ["sioc:Usergroup", "redaktor:Instance"]} :
The sioc:Usergroup is a redaktor:Instance too because in system:redaktor some things might be different from other systems:
A user can use different clients, the instance can serve different clients and the instance is an Actor too.
I think that the skohub approach / workflow has lots of benefits for collaboratively publishing Vocabularies from a single repo and you can even follow …
[edit / addendum to anyone never heard of SKOS or skohub]
OWL and SKOS are the W3C recommendations.
The skohub.io people are powering a large portion of the infrastructure for german universities, e.g. the System where I find the books in uni libraries nearby, they know what they are doing - to learn about the benefits watch the short Demo Video or Conf Video …