“Search the Cabal Defense Net in the observation tower for the location of the Vex Spire.”
Head over to the right as you exit the tunnel. This will put you on the ridge that will lead directly to the Cabal Tower that you want. Expect to encounter some resistance in terms of Legionaries and Phalanx. There are also likely to be a few Psions as well.
Head up the ramp you see and over into the middle of the tower. The next part will start immediately after you Deploy Ghost. Keep on guard as you do. A Centurion likes to show up around this time. As this part finishes, you will likely also encounter a Colossus