Crown vetch is certainly among a long list of non-native plants that has caused more problems than it solved. If you have crown vetch in with other plants, you might try a nonselective herbicide and paint it on with a brush so you do not kill the other plants with overspray. You will have to do this repeatedly and be vigilant about keeping up with it. If the crown vetch is invading an area that you can do without for a year, you could try solarization. Solarization utilizes black plastic in maximizing the heat of the sun to sterilize the soil. It must be done in the summer for the area to become hot enough to help. Start in the late spring by watering in the area that is to be treated quite well. Then cover the area with black plastic and secure the edges with soil, rocks, or earth staples (you can make them with wire hangers). Leave the plastic on for 60 days. Everything in the soil will be sterilized and you should be able to start over. Even with all of this, if crown vetch and it grows near you, it may always be something you need to stay on top of.