AFAIK the Vasudans are mostly concerned about a Shivan invasion, and don't want to waste their forces on something they consider a domestic Terran issue.
Also, the Zods change their opinion towards the end when this actually happens.
It's not a "domestic Terran issue" when the EA are literally bombing your capital RIGHT NOW
. Again, an alliance is more than pinky-swearing to be friends. If the GTVA is a real thing and not just for show, if there are joint governmental and command structures, then thousands if not millions of Vasudans in the system and especially on Janus are dead
. The EA have invaded Terran-Vasudan space, destroyed Vasudan property, and slaughtered Vasudan civilians. The Vasudan Imperium must
act to ensure its own survival. If the Emperor won't, someone further down the chain will. Every war hawk politician and ambitious general in Vasudan space must be drooling
at this point, as the EA have handed them a nigh unassailable case for war.
Remember that in BP, the GTVA was trying to conquer the UEF, who really had nothing against them and were happy on their own (at least from the perspective of UEF citizens unaware of the whole psychic alien business). The EA at least boast about their intentions to conquer the Terran half of the GTVA, and once they have that, they would have an overwhelming economic and military advantage. The best interests of the Vasudan people and Vasudan state in Inferno are that the Earth Alliance be destroyed, or at least contained.