I was playing through the story missions with my hunter character yesterday, and the Ghost mentioned that it was during the collapse that the Vex turned Mercury into a machine and only the intervention of the Traveller stopped them from converting the other worlds. This means the Vex were active during the collapse, and destroyed at least one human colony world (it has been mentioned that during the Golden Age the Traveller had turned Mercury into a garden paradise). We don't know if the Vex worked with the Darkness (the potential account of the collapse by what is possibly Rasputin would suggest no one worked with the Darkness), if they were working against the Darkness (the Exo Stranger describe the Vex as being so evil that they would fight against other evil), if they were independent of the conflict with the Darkness and were using the Collapse as a distraction to seize worlds, or if the Vex are the Darkness. The Vex structures on Venus predate humanity by billions of years. During the Collapse they were taking entire worlds away and managed to convert one into a machine in a short amount of time. The Vex are potentially inside of every world of the solar system. And the only thing that stopped them in the Collapse was the power of the Traveller. I would say the full might of the Vex was used during the Collapse, but is dormant before it and ever since. We're seeing a resurgence in the game, but the Guardians and Ghosts (the last embers of the Traveller's power) are managing to keep them in check.