I've been taking vitamin D for over 3 years and have only just (literally since last week) developed nails that actually grow. For the first time in my life they have stopped splitting and, if they have grown to any length, have always broken at the slightest tap (e.g. on the dashboard when I go to grab the steering wheel.) I have even had to cut them! For all my life, I have worn gloves and spent a fortune on all sorts of creams, oils, powders, buffers and varnishes to try and protect them, but to no avail.
I now have to wait until September to see if they don't break off, split and just erode down to the quick.
The sun and vitamin D therefore seem to be the only answer, but it takes time. Best start now with the supplements and/or move to the Canary Islands.