Don't forget that dog foods are cooked at a very high temp and most of the vitamins break down before your doggy gets to finally eat them..
If you look here, there's a multivitamin, with 200iu of vitamin A and dogs have to take from 3 to 5 of these... So it looks like no matter what, they need tons...
http://www.healthspan.co.uk/shop/product.aspx?id=MV-D