Good grief! Northern European countries have solved this problem long ago - Moms just take a teaspooful of Cod Liver Oil daily and Vitamin D problem is solved. The Norwegians make the best oil and you can buy it in a number of flavors.
You can measure a "deficiency" but did the babies just have low levels of vitamin D, or did they have any bone or growth problems?
Mrs VS
tsk tsk, if you go around posting common sensical things like that folks around here will break into a nervous sweat and start screaming about overdosing.
Or rub a little oil (regular kind like for massage, NOT mineral oil) on baby's skin and let him in the sun for a little while. Even a short sun exposure a few times a week will do it. Even less will do. Then don't wash the exposed skin for a few hours.