app.html

<div class="col-md-4 col-xs-12">
  <div class="form-group" v-bind:class="{ 'has-error' : checkFieldError(pacchetto, 'nome') }">
      <label class="control-label">Nome</label>
      <input type="text" class="form-control" v-model="pacchetto.nome" />
      <div class="help-block" v-html="textFieldError(pacchetto, 'nome')"></div>
  </div>
</div>

app.js

checkFieldError: function(model, field)
{
    return model.__errors && model.__errors[field] != undefined;
},
textFieldError: function(model, field)
{
    return model.__errors && model.__errors[field] != undefined ? model.__errors[field] : '';
},

server.php

<?php
return [ '__errors' => $model->firstErrors ];
?>

sommario.html

<div class="alert alert-danger" v-if=" '__errors' in model && Object.keys(model.__errors).length > 0 ">
    <ul>
        <li v-for="err in model.__errors">{{ err }}</li>
    </ul>
</div>