CComment docs

VirtueMart – com_virtuemart (Pro feature)

Go to your Joomla backend components->CComment-> settings and click on new. From the dropdown select com_virtuemart and click on next. Save the configuration.

Now go to the Plugin Manager (Extensions -> Plugin Manager) & find the Content - CComment plugin. Open it, enable support for virtuemart in it & make sure that it is published. Now when in Virtuemart on the product page you should see the comment form.