Oh yes, been through this one...
I'd cut down on the portion size you give her - so 1 bite size thing of everything - literally, one piece of carrot, 1 small (dice size?) piece of meat, 1 cut up small potato. Then tell her she can either eat it and she'll get a treat after, or she doesn't have to, but no treat.
If you're giving her a vitamin and she's having a good breakfast she'll be fine. It doesn't matter if she doesn't eat any tea for a few days.
If she eats the lot regularly, slowly increase the amount you put on her plate.
Whatever happens, try not to make any fuss about it (the tricky bit! ). Let her leave the table whenever she wants - the rules are that when she's left, that's it - she can't come back (we always scrape the plates as soon as our two have left) and if she hasn't eaten all/what you consider to be enough, then there's no treat.
Make it black and white, no messing, no negotiating. But the tough thing is being prepared to see her eat no tea for a few days. You'll definitely win this one, honest!!