Hello Nina,
If you make this change you would just need to keep in mind that you would need to backup this file in any future Vertex update.
You can change it but the proper syntax would be:
window.open('
http://www.yoursite.com')
changing:
onclick="window.document.location.href='<?php echo $s5_lr_tab1_click; ?>'"
to the below should do it:
onclick="window.open('<?php echo $s5_lr_tab1_click; ?>')"