Virility is its own stat but it also runs parallel to sheer sperm output. The more your character cums in volume, the more likely it is for impregnation to be successful. Check that you don't have the "sterile" perk active by accident (it prevents impregnation 100% on part of the penis haver). Oh, you can make enchanted gear for that purpose, by the by - fertility and virility as well as cum output-enhancing items, if you want to be absolutely certain.
Usually a cum output in the 5L range does the trick just fine. That's also about when the output is strong enough to break through sterility pills.