Single product menu items do not work if SEF is enabled in Joomla settings. This works perfectly in Joomla 3.10.10.
It has been tested with the template with a third party template and with the virtuemart core template and with neither it works in Joomla 4, instead it shows the main page of the products.
System information:
Joomla 4.1.5, Virtuemart 4.0.2 10661, PHP 7.4.20
There are issues with the J4 router which are not yet fully understood.