Vector layer disappears when switching to OSM layer...

In the following code if I use the WMS layer my polygons (the geos variable) are displayed properly. If I switch to the OSM layer the OSM layer displays just fine, but the vector layer disappears from the map (despite being in the layers collection. Any ideas?

var options = {
                projection: "EPSG:4326",
                displayProjection: "EPSG:4326",
                numZoomLevels: 20
            };
            that.map = new OpenLayers.Map('map', options);
//            that.layer = new OpenLayers.Layer.WMS("OpenLayers WMS",
//                "http://vmap0.tiles.osgeo.org/wms/vmap0",
//                { layers: 'basic' });

            that.layer = new OpenLayers.Layer.OSM({ opacity: 50 });

            var featurecollection = {
                "type": "FeatureCollection",
                "features": [
                    { "geometry": {
                        "type": "GeometryCollection",
                        "geometries": geos
                    },
                        "type": "Feature",
                        "properties": {}
                    }
                ]
            };

            var geojson_format = new OpenLayers.Format.GeoJSON();            
            var vector_layer = new OpenLayers.Layer.Vector();

            that.map.addLayers([vector_layer, that.layer]);

            vector_layer.events.register("featureadded", that.map.layers[1], that.zoomToLeftHalf);
            vector_layer.addFeatures(geojson_format.read(featurecollection));