class VirtualPage {
/**
 * @var int $page_id    The ID of the page where you would want to inject your custom template.
 **/
var $page_id = 0;

/**
 * @var (mixed) $callback   The callback function/method that will be called to replace the current post.
 **/
var $callback = false;

function __construct( $page_id, $callback = false ) {
    $this->page_id = $page_id;
    $this->callback = $callback;

    /**
     * Set the injector when there are posts found.
     **/
    add_action( 'posts_results', array( $this, 'posts_results' ), 10, 2 );
}

function posts_results( $posts, $wp ) {

    if ( $wp->is_main_query()
        && $wp->is_singular
        && count( $posts ) > 0
        && $posts[0]->ID == $this->page_id ) {

        $found_posts = count( $posts );

        /**
         * $wp->post_count holds the number of iterated posts. We'll make WP believe that all
         * posts are iterated.
         **/
        $wp->post_count = $found_posts;

        /**
         * $wp->current_post holds the current post index. Setting it to the last post index
         * will immediately trigger `loop_end` action hook.
         **/
        $wp->current_post = $found_posts - 1;

        add_action( 'loop_end', array( $this, 'loop_end' ) );

        /**
         * Immediately remove the hook!
         **/
        remove_action( 'posts_results', array( $this, 'posts_results' ), 10, 2 );
    }

    return $posts;
}

function loop_end() {
    if ( $this->callback ) {
        // Call your callback here or do your stuff here
        call_user_func( $this->callback );
    }

    /**
     * Immediately remove the hook!
     **/
    remove_action( 'loop_end', array( $this, 'loop_end' ) );
}