Example vega.json file that doesn't work

vega.json

{
  "axes": [],
  "data": [
    {
      "format": {
        "feature": "world-countries",
        "type": "topojson"
      },
      "name": "countries",
      "transform": [
        {
          "projection": "winkel3",
          "scale": 200,
          "translate": [
            480,
            250
          ],
          "type": "geopath",
          "value": "data"
        }
      ],
      "url": "vincent_map_data/world-countries.topo.json"
    }
  ],
  "height": 500,
  "legends": [],
  "marks": [
    {
      "from": {
        "data": "countries"
      },
      "properties": {
        "enter": {
          "path": {
            "field": "path"
          },
          "stroke": {
            "value": "#000000"
          }
        },
        "update": {
          "fill": {
            "value": "steelblue"
          }
        }
      },
      "type": "path"
    }
  ],
  "padding": {
    "bottom": 50,
    "left": 50,
    "right": 100,
    "top": 10
  },
  "scales": [],
  "width": 960
}