Try changing your ajax call with the following code,
var vehicle = $("#ModelResult").val();
$.ajax({
type: 'GET',
url: '../../VehiclesController/GetVehiclePart?id=' + vehicle,
dataType: 'json',
success: function (data) {
vehicle = $("#ModelResult").val();
console.log(vehicle);
},
async: true
});
[Edit]
Added dataType twice :doh:
-KR