Janus: We know Janus was willing to do anything to save the humans, which leads me to the theory that they might have landed on a Vanduul planet. In a desperate struggle to save the humans, Janus terraformed the planet, killing the Vanduul on it. This would explain why the Vanduul don't exactly love us.
Terraforming is a long process requiring a lot of resources, not something that can be done on short notice. The Vanduul just don't seem to like anyone very much (although they do seem to have a bit more of a mad-on for Humans than the other races).