PhilM, you can always add vector to an image as a clipping mask. Or what did you mean?
But the option you explained looks like a bug. It works in some strange way and don't follow to global logic of Clipping Masks.
Here is example:
Drag vector layer inside Adjustment Layer. Now it behave like inverted clipping mask, but shows normal eye icon instead of clipping mask icon and don't have Clipping Mask options in Layer Attributes.
If i convert it to clipping mask there, it simply disappears and don't works.
If i drag it outside Adjustment Layer and then drag back inside Adjustment Layer - it now works as normal Clipping Mask.
UPD. As Martin Huber explained to me - mask don't works because additional "Layer Mask" menu toggle was not automatically enabled. To be honest it is really hard to imagine real life situation when "Clip" is enabled but "Layer Mask" is disabled for purpose. Also there is a toggle "Invisible" which behave exact same as disabled "Layer Mask". Too many complex options for so simple tastks.