What evidence is there that there were no Vasudan defectors or double agents, especially since one of the themes of the FS universe is that Terrans and Vasudans are much more alike than either would care to admit.
My thought is, if either race were to glass a planet, there would be defection in droves. For example, if the Terrans nuked Vasuda, many Terrans would defect and bring a lot of technology to the Vasudans. At the same time, the Vasudans would band together and any defections from them would abruptly stop. So while we may have the technology and potential to destroy the surface of entire planets, what's the chances we'd actually stoop to using it?
What makes you think the majority of Terrans in 2335 give a rat's ass about Vasudans, or vice versa? The PVFr Bast tech description shows that the GTVA has no problem with the wholesale massacre of Vasudan civilians in unarmed shipping vessels. After fighting a war with an enemy for 14 years, hating them becomes natural and inevitable. The fact that Vasudans are not human makes hatred even easier. This sort of stuff happened all the time in human-human conflicts (see: conquistadors, Tamerlane, the Bible's description of ancient Middle Eastern campaigns, the sack of Carthage in the last Punic War, the sack of Jerusalem in the crusades (the Pope gave the order "kill them all; God will know his own", the origin of the phrase "kill 'em all and let God sort 'em out"), the Scramble for Africa, the Indian Wars, the Rape of Nanking, the Holocaust, the My Lai Massacre, the Darfur genocide, the Kosovo ethnic cleansing, the Belgian Congo...), it would be even more likely in a human-alien conflict.
The average human in 2335 would probably consider the mass murder of Vasuda Prime's population a glorious victory. Humans are not naturally tolerant and compassionate to people outside the "in-group", and encouraging people to feel any trace of empathy for the Vasudans would be against the GTA's strategic interests, while encouraging them to hate Vasudans would further their long-term goals.
The idea of civilians being a protected class of innocents is an idea only around 200 years old that was created in a specific cultural and sociological milieu with a specific sort of enemy in mind (the laws and customs of war have been frequently ignored by states that nominally recognized them when fighting an enemy they really hated--the Germans in WWII for instance, extended Geneva/Hague Convention protections to enemies on the Western Front, where white Western Europeans more or less conforming to "master race" specifications lived, but not the Eastern Front, where a variety of "life unworthy of life" ethnic groups lived). Widespread murder and pillage up to and including genocide is the rule in warfare, not the exception.