Chuckie,
If you have a threat of crabgrass ruining anything you want to plant, you have some options. There are crabgrass specific pre-emergent herbicides like Balan that will nip the whole crabgrass problem in the bud pretty well. Out west anyhow, it's best to put down a crabgrass pre-emergent sometime in February to stop any seed from establishing itself. A couple seasons of this and you can get a real good handle on crabgrass. It's not going to be cheap, but neither is seeding something in and having crabgrass take over.
Also, if you have the same type of vetch that we do, then they prefer to eat it when it's dried out a tad. If they haven't got on it yet don't give up hope, look in about 6 weeks again when it's drying up and you may find them eating it real well.