We only seem to be fighting them because some mysterious shifty-eyed exo told us to. We chased them down because we were told to. It seems to that while the guardians are necessary to destroy the darkness (because of their light), the vex are buying us the time that we need to reclaim the strength that we lost. Maybe Rasputin knows that us guardians need to think that we are alone in this fight, so that we don't place all of our faith in the vex to save us.