That's a pretty good point. To be honest, with how efficiently they can transform planets and how little we've done to actually stop them, would explain why they don't care enough to fight us off. Especially since we basically helped the Vex when we destroyed Atheon, the only enemy of the Vex network that actually has the knowledge to possibly defeat them. Really, the worse thing we've done to the Vex was screw around with an extremely small number of their simulations. Considering how many thousands of millions of simulations they run, popping in and making a few unusable is not going to hinder their research.