Grace was a slow gainer too, didn't hit 20 lbs until 17 months. At almost 4, she still only weighs 33 lbs, but that is perfect for her, she's just a petite child. (I have two tiny sisters, so it runs in my family) Her doctor really pushed me to try to get her more fat in her diet, but I had to listen to my mommy instinct too which said she was getting enough. She slept well, played hard, and learned well and exceeded her milestones.
If your dd is still nursing 3x per day and getting that much yogurt and cheese, I would think that is plenty on top of a good solids diet. The thing about 1 yr olds too is that they start refusing food - oh is that fun - not! They get so darn picky. So get her veggies and meats into her while you can. At 13 months old, Grace had stopped nursing (I was 24 weeks pregnant), and she at three meals and two snacks per day. So if your dd is eating 3 meals per day and 3 nursings, that seems perfect. As long as your dd nurses, I probably wouldn't bother with vitamin drops, she is still getting plenty of what she needs from you. Unless your ped tests for iron and it is low, I wouldn't bother with iron drops either. They get so much food that is iron fortified, cereals and grains.
I don't have any yogurt in the house right now, so I can't check for vitamin content. I'll try to remember to look when I go to the store later.