Hii Paola,
this case must be specific to your website. I can't be sure what you did to setup those variants. But as you can see on the live demo:
demo.flexiblewebdesign.com/materialmart/index.php?option=com_virtuemart&view=productdetails&virtuemart_category_id=10&virtuemart_product_id=106
once you select a variant (color or size), the child product gets loaded and the value in the dropdown also gets selected respectively.
Are you sure you disabled the jQuery chosen() from VM Configuration?
- Navigate to Joomla Backend >> Virtuemart Configuration.
- Click "Templates" Tab
- Find this option: "Use jQuery chosen for dropdowns in FE"
- Make sure it is disabled
Regards.