Even if your vigor is on the backbar, once you go to your front bar nirnhoned helps on that.
Just use the uesp build editor and compare effective weapon damage. Sharpened beats nirnhoned on 2h maces, but i think that's it.
It depends on your build. The key difference is that Nirnhoned is a FLAT bonus. Sharpened affects all your damage sources.
If you are running low max stat and weapon/spellpower, the additional damage from Nirnhoned will be noticeable. Sharpened requires you to already have good damage and will make it so you keep that damage through armor.
In that regard, you best test it yourself. I guess most damage-oriented builds will see more use from Sharpened. I excluded healing strength, exotic set and mundus combinations, and so on here to keep it short.
@NuarBlack @Lord-Otto
Shameless self-plug here, but the calculator linked in my sig is designed to calculate exactly how much of a net % DPS increase you'll gain from any added amount of penetration (taking into account your baseline penetration as well as your targets armour).
To figure out the opportunity cost vs. nirn, you would then compare it to the % DPS gained from the WD/SD of nirn, which you can calculate over at the UESP skill browser:
https://esoitem.uesp.net/viewSkills.php
Two very helpful tools that I utilise heavily when theorycrafting