By the way, if it was Vex who killed, not Shael, then Vishmaster's confidence in Nefela being a Mafia member is justified.
And why would the Mafia kill with Vex? If it was Shael, then it's clear: she's already likely to be eliminated, so the Mafia isn't afraid that anything will happen to her. If it was Salamander, that would also be justified. But why Vex?