tile.js

var vt = new mapnik.VectorTile(z, x, y);

// Get data in the tile
var stream = collection.find({ ... within query ...}).stream();

stream.on('data', function(data) {
  vt.addGeoJSON(JSON.stringify(data.feature), 'my-layer');
});

stream.on('end', function() {
  // should probably stream the results back for more efficiency
  var tile = vt.getDataSync(); 
  res.send(tile);
});