[...]
Anyway, a small, non-exhaustive sampling with rough, incomplete notes -
for the sake of speed:
Thanks!
I may be missing something here, but none of the examples you gave show any signs of being derived code. They use vty.[ch], but do not include any of their code (bear in mind that I'm not familiar with either code base).
Being merely dependent on third-party code is not, to my understanding, sufficient to be considered derived code.
It looks like this is a clearcut case of aggregation:
somepackage/gpllibrary/COPYING.GPL
somepackage/bsdlibrary/COPYING.BSD
somepackage/COPYING.GPL
bsdlibrary can be distributed on its own under the terms of the BSD, but if it's distributed along with gpllibrary, then the combined licensing terms of both libraries can only be met by the combined package being distributed under the terms of the GPL.
In FRR's case, the combined repository is clearly labelled as being distributable under the terms of the GPL. This all looks fine to me.