Isn't PolyViSol an iron supplement? Maybe they also make other supplements for vitamin D? The one we bought was an iron supplement.
In any case, our (first) pediatrician recommended that we give an iron supplement to my daughter since we were not feeding her cereal at 4 mos. My partner picked the stuff up at the store but, after doing some research on our own, we decided not to give it to her.
From what I've gathered from other moms, there seems to be a widespread belief among pediatricians that iron supplementation is necessary when a child is not on a fortified formula or cereal or whatever.
The problem is that lots of times, iron levels aren't even checked - they just assume that since you're not feeding her anything "fortified" then her iron stores will be depleted. Research shows that it's just not true and even IF levels fluctuate, there's no danger.
You're probably familiar but www.kellymom.com is an awesome searchable resource for BFing moms. Even though my daughter is weaned, it's still my first stop anytime I'm doing any research.
Anyway, here's what she says about supplementing:
http://www.kellymom.com/nutrition/vitamins/iron.html
Good luck!
J.