Order custom fields alphabetically

It looks like you’re trying to output the wrong stuff. You’ve ordered the $post_objects, not the $vMarca. Try using the $post_objects variable for your if and foreach statements thus:

<?php 
    $vMarca = get_field('tienda_marcas'  , false, false);
    $post_objects = get_posts(array(
        'post__in'   => $vMarca,
        'order'   => 'ASC'  ,
        'orderby'   => 'title'  
    ));

    if($post_objects)
    {
        echo '<ul>' ;
        foreach($post_objects as $post)
        {
            setup_postdata($post); ?>
            <li><a href="https://wordpress.stackexchange.com/questions/91370/<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
            <?php the_content(); ?>
        }
        echo '</ul>' ;

    ?>