[Vuejs]-Ajax self invoked function to vue data

Instead you can perform the ajax call in the created() lifecycle hook and set the data property modello to the response there like this:

var Metromappa = new Vue({
            el: '#metromappa',
            data: {
                modello:null
            },
            methods:{

            },
            created(){
                var self = this;
                $.ajax({
                    type:'get',
                    url:'https://api.myjson.com/bins/a8ohr',
                    dataType: 'json',
                    success: function(response){
                        self.modello = response;
                    }
                });
            },

        })

Here is the jsFiddle

If you want to seperate the logic:

var getData = function(){
            return $.ajax({
                    type:'get',
                    url:'https://api.myjson.com/bins/a8ohr',
                    dataType: 'json',
                    success: function(response){
                       console.log(response);
                    }
                });


        };




          var Metromappa = new Vue({
            el: '#metromappa',
                data: {
                    modello:null
                },
                 methods:{

                 },
                 beforeMount(){
                     var self = this;
                    getData().then(function(response){
                        self.modello = response;
                    }, function(error){});

                }
            })

here is the updated fiddle

thanks to Bert Evans for pointing out my mistake