Mithrax shares what he learned by studying the Vex data you brought him: the City is a conflux, a paradromic knot of threads from the Vex network. The Vex are draining the energy from the City, but beyond that, their motives are unclear.
There is no way to easily separate the City from the Vex domain—for one to survive, the other must be destroyed.
But he sees hope in the future, as your victories against the Vex are visible in the simulated sky. He will continue his studies, upgrade your Splicer Gauntlet, and learn how to break the City free once again.