[Vuejs]-Load a combo with AJAX in VueJS (onLoad)

The scope of this is not what you think. In your case, this is the jqXHR object of the Ajax call, not the Vue instance.

You should assign this to a support variable:

 getFederations: function() {
   var url = '/api/v1/federationsofcats';
   var myVm = this;
   $.getJSON(url, function (data) {
        console.log(data);
        myVm.federations = data;
   });
 }