Remove Custom Post Type (name/slug) from permalink url

functions.php

/**
 * Remove the slug from published post permalinks. 
 * Only affect our CPT though.
 */
function vipx_remove_cpt_slug( $post_link, $post, $leavename ) {
 
    if ( ! in_array( $post->post_type, array( 'event' ) ) 
        || 'publish' != $post->post_status )
        return $post_link;
 
    $post_link = str_replace( 
        '/' . $post->post_type . '/', '/', $post_link 
    );
 
    return $post_link;
}
add_filter( 'post_type_link', 'vipx_remove_cpt_slug', 10, 3 );

/**
 * Some hackery to have WordPress match postname to any of our public 
 * post types. All of our public post types can have /post-name/ as 
 * the slug, so they better be unique across all posts. Typically core 
 * only accounts for posts and pages where the slug is /post-name/
 */
function vipx_parse_request_tricksy( $query ) {
 
    // Only noop the main query
    if ( ! $query->is_main_query() )
        return;
 
    // Only noop our very specific rewrite rule match
    if ( 2 != count( $query->query )
        || ! isset( $query->query[ 'page' ] ) )
        return;
 
    // 'name' will be set if post permalinks are just post_name, 
    // otherwise the page rule will match
    if ( ! empty( $query->query[ 'name' ] ) )
        $query->set( 'post_type', array( 'post', 'event', 'page' ) );
}
add_action( 'pre_get_posts', 'vipx_parse_request_tricksy' );