Since derivative distributions (in the loose meaning of the term) might
be simple overlays on top of the stock debian distribution (e.g.
http://neuro.debian.net, http://deb-multimedia.org), current approach of
dpkg-vendor with a single 'default' symlink pointing to the "most-derived" one
would not scale, happen  multiple of such overlays used possibly providing
their own origins definition.

ATM those origins definitions (unless accessible from the 'default') seems to
be accessible only by explicitly specifying --vendor.  Would there be much of
harm if all files (e.g. having '^Vendor:' and not having any suffix/extension)
would be loaded by default.  Then logically --is and --derives-from queries
would work for those overlayed origins.

And use of 'default' symlink should overall be deprecated then I guess --
inheritance should be obvious from the full set, assuming that Parent:
definitions still define a DAG (or may be even restrict to a tree?) ;)