I've always been curious if they were a failed attempt at the exos from another dimension. The exos were made as inhuman war machines designed to counter enemies of the darkness, but from what I read in cayde's journal it sounded like the exos came before the vex, prior to the collapse. But being that the vex are technically not born of darkness it seems plausible. I also wonder if the vex predicted the tear in a simulation relative to the one they made for the Ishtar academy scientists? This they could've planned for the invasion to take place.