At this point in time, I noted that the issue about Vampire Knight retinues being invisible seems to only apply to certain cultures whose characters have been turned into the von Carstein or Blood Dragon strain of vampires.
In the example I used for myself, a Strigany/Strygan character who is a von Carstein is not able to produce Vampire Knight retinue units that are visible on the map, while changing his culture intentionally to a Bretonnian or Imperial culture group, it becomes visible again.
Since this appears related to map character models, I take that the vampire retinue does not draw on the in-game resources fully yet?