As a matter of fact, one can use various services to extract OSM vector data. Have a look at http://export.hotosm.org (my favorite), or http://www.geofabrik.de/data/shapefiles.html, or http://overpass-turbo.eu/ or other services (as far as I can remember, QGIS can also be used to donwload OSM data). However, the result will be a polyline instead of a polygon, but I think it's a start