Make sure you are doing the modification in the correct PHP file.
If you are using "default" layout for product details page, then the PHP file is this:
/templates/YOUR_TEMPLATE/html/com_virtuemart/productdetails/default.php
If you are using "flexv2" layout for the product details page, then the PHP file is this:
/templates/YOUR_TEMPLATE/html/com_virtuemart/productdetails/flexv2.php
I am sorry but i can't provide you any further instruction but my suggestion in my 1st post should be working just fine.
Of course don't forget to move the
class="selected" to the one that you want to show off by default.
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Cheers,
Support