Does your virtual_feature.pl file have a feature_webmin function? This is needed to grant domain owners access, like so : https://github.com/virtualmin/virtualmin-awstats/blob/master/virtual_feature.pl#L595
Note that if you add this during development, you may need to do a one-time re-generation of available modules for a domain’s user by running :
virtualmin modify-domain --domain whatever.com