Smooth Vetch is very similar in appearance to Vicia villosa (Hairy Vetch), and some authorities consider it to be a variety of the latter species. The stems and leaf stalks of Smooth Vetch are hairless or sparsely hairy; in the latter case, these hairs are very short, fine, and appressed against the stems or stalks. In contrast, Hairy Vetch has long spreading hairs on its stems and leaf stalks. Another difference, in my experience, is the color of their flowers: Smooth Vetch usually has pink flowers, while Hairy Vetch usually has blue-violet flowers.
This plant is a winter or spring annual that becomes 1½–3' long. It has a tendency to sprawl in the absence of supportive vegetation. The stems are light green, terete, ridged, and shiny; they often have sparse fine hairs that are very short and appressed in the channels between the ridges. The alternate compound leaves are evenly pinnate with 12-20 leaflets and a pair of terminal tendrils that can cling to adjacent vegetation. Each leaflet is up to 1" long and ¼" across, light to medium green, oblong, and smooth along the margins.
This plant might be poisonous
It can be invasive in Alaska, Florida, Georgia, Kansas, Michigan, Minnesota, Oregon, and Washington state — as well as in Japan and Europe.