Crown vetch is very poor for erosion control. If you are OK with a little height (not a ground cover), then you could use amur silvergrass, but it is invasive and must be kept mowed at the perimeter to stop it from spreading by roots (check local laws on planting). Ours does not spread by seed, but it is a sod former and out-competes most things. A native prairie grass (or mix) could also work but it will need to be burned ever 3 years or so exposing the ground to erosion for a short time. In any case, old stumps will likely re-sprout unless you treated them with a herbicide like Tordon soon after cutting. Crown Vetch and myrtle are also considered invasive exotics. You must kill existing vegetation to get a good stand of most prairie grasses.