How to use angular-google-maps <markers> directive?

As you figured out, it's stated in here: http://angular-ui.github.io/angular-google-maps/#!/api/markers that the value of the property coords must be between quotes.

Also, in v1.1, for each marker you need to define an id (the name of the id property is given by idKey), which defaults to model.id,

so, in your case, and for it to work now it would have to be

<markers models="vehicles" coords="'last_known_location'"></markers>

and the vehicles array, for instance:

$scope.vehicles = [{
  id: "first",
  stuff: "stuff",
  last_known_location: {
      latitude: 37.3694868,
      longitude: -5.9803275
  }
}];

(Notice the id property)