Hello Osckar,
You do not need any special PHP extension for this plugin, your problem may be from web server settings or PHP settings. Some permistions may be set. Give me more info about these problem to check it out. Thanks.
You may use the following code:
<?php echo do_shortcode('[vasaioqrcode s="600" x="q" t="gif" b="3" c="000000" p="0" m="hahaha" ]'); ?>
I will make a function for this.
