REST API - is it possible to create users?

No.  The REST API focuses on object operations.

It is possible to extend the REST API using vault extension methods (which could, then, create users), then call those through the REST API, but out of the box it does not support these sorts of operations.

No.  The REST API focuses on object operations.

It is possible to extend the REST API using vault extension methods (which could, then, create users), then call those through the REST API, but out of the box it does not support these sorts of operations.