But why would she want all the Vix dead? They're the good guys!
I sincerely enjoy thinking about Vix as the good guys from The Sundering. Or at least Joran perfectly fits the picture - honourable, unforgiving, self-righteous dick.
Honestly north holds a huge advantage right now. 1/3 more nobles, potentially 1/2 more CS available on short notice. But most importantly with the inclusion of GX nobles north started to finally work cohesively! No more strategic blunders.
Even more so, three of the southern realms are a mix of just plain underpopulated and long travel times making their poorly led military help of little consequence. Plus at least one is just plain barbaric.