Interestingly if you read the fragment regarding the cabal a stowaway ghost reveals that there are divided loyalties to a distant empire that potentially is destroyed (the holographic planet fissuring) and it feels as though they are running away from something. We don't know how far the vex empire reaches but unless they are many galaxies wide I don't think the vex are what the cabal are running from. I think it may be more likely that the cabal set up fortifications on Mars and as they began to "dig in" they tampered with the Vex network and set in motion defense protocols which led to their current war. Whatever may be causing them to run will likely be a much larger part of the story later on likely in Destiny 2 or 3