Crown vetch (Coronilla varia) is often used as a ground cover on steep roadside banks to hold soil in place and cut down on erosion. Although it may serve a good purpose along a roadside where it can be mowed to keep it in bounds, in a backyard it can become invasive. I'd spray it with glyphosate (Roundup) if possible. It may take repeat applications to kill the extensive taproot of this legume. Otherwise, keep pulling it. I planted one plant 10 years ago and am still finding stragglers trying to survive in my garden.