example.php

<?php
// Just set up an array and pass it to the function, and
// echo out the returned HTML.

$atts = array(
    'link'  => $videourl,
    'thumb' => $videothumb,
);

echo themeblvd_shortcode_lightbox( $atts );

// And you can simplify it by just passing the array directly
// to the function, also.

echo themeblvd_shortcode_lightbox( array(
    'link'  => $videourl,
    'thumb' => $videothumb,
) );