I’m very much for the possibility of adding vocabulary extensions and even more so for making vocabularies (and everything) content-addressed.
However, I feel that the way this allows extensions is very limited. This FEP (and the original proposal by @cwebber) allow extension terms - single terms that are content-addressed. A vocabulary, on the other hand, is a collection of terms. I’m convinced that any slightly more complex extension will require multiple terms that potentially refer to each other - a collection of terms, a vocabulary.
I think it would be better if an entire collection of terms with their human readable (and possibly multi-lingual) descriptions can be content-addressed.
I’ve brought up the same objection to the original proposal (Content addressed vocabulary for extensions).