<?php
$args = array(
    'post_type' => 'myportfoliotype', // Custom taxonomie name
    'posts_per_page' => '15', // Number of Posts to display
    'tax_query' => array(
        array(
            'taxonomy' => 'specifics', // Custom taxonomy name of your tags
            'field' => 'slug',
            'terms' => 'Vermittelt', // Your term
            'operator' => 'NOT IN' // Exclude your term or include with IN
        )
    )
);

$my_query = new WP_Query( $args );

if ( $my_query->have_posts() ) : while ( $my_query->have_posts() ) : $my_query->the_post();?>
    <?php get_template_part('/includes/folio-types/foliopost-home');?> // Template to used for your post slider
<?php endwhile; endif; ?>