vector-proxy.js

var express = require("express"),
    cors = require("cors"),
    request = require("request"),
    path = require("path"),
    app = express();

app.use(cors());

app.all("*",function(req,res){

  // Get the vector tile from Mapzen
  request.get("https://" + path.join("vector.mapzen.com",req.url),function(err,r,body){

    res.status(r.statusCode);

    // Quit if error
    if (r.statusCode !== 200) {
      return res.send(body || "");
    }

    // This assumes TopoJSON, could modify for GeoJSON, etc.
    var topo = JSON.parse(body);

    // Modify the content
    if ("buildings" in topo.objects) {
      topo.objects.buildings.geometries.forEach(function(geom){

        // Add/edit properties

      });
    }

    // Send the modified JSON
    res.json(topo);

  });

});

app.listen(8888);