It's a PITA for NewsForge admins to have to select NewsVac plus
whatever other topics for their vac'ced stories, and then disable all
other nexuses so that it only appears in NewsVac.
The natural solution would seem to be creating a new weight, say
0.5, which is "NewsVac Only". Give a topic that weight and the
topic_parent min_weights would ensure that it only propagates up
into the NewsVac nexus, no others.
I *think* this would only mean editing the template with the
"Sectionworthy/Mainpageworthy" menu, since the system is
set up for floating-point weights. Have to test it first though.