Would it be possible to add a doc USE flag to vmpk which controls whether manpages are installed and therefore allows to ignore the docbook dependency?
I'm not good at writing ebuilds but that should be easy enough for the maintainer:
if the doc USE-Flag is NOT set, then
sed -i /add_subdirectory (man)/d CMakeList.txt
Normally man pages are always installed. One way to prevent compiling them is to distributed pre-compiled files and unpack and install those, instead.
We always install manpages, and in this case they do not even cause additional deps.