The vitamin D available in breastmilk is extremely bioavailable, meaning that your baby/child can actually absorb it! Plus, when you are pregnant, your baby absorbs extra stores of vitamin D that typically lasts them at least 6-9 mos (right around the time that they start to eat solids & get more vitamins from that).
In addition, your body stores vitamin D from the sun, so as little as 10-20 min a day of sunlight provides plenty of vitamin D & since it IS stored, it lasts for months in the body!
There is TONS of info about vitamin D on www.KellyMom.com
I, personally, have never heard of a case of Rickets in a breastfed baby unless the mother was completely malnourished.
That all being said, I do, on occasion, give my son an all-natural, plant-based mulit-vitamin. We really like the Animal Parade brand, which you can find at any health food store. My 5 year old really likes their chewable vitamins too & since they are plant-based, their body can, again, actually absorb the vitamins!
Good luck!