Part of the problem with giving them seeds (even if you are adding vitamins) is that seeds tend to be high in fat that they don't work off by flying the way that they do in the wild which is why most owners prefer pellets. Also, it's always better to get vitamins naturally than from a supplement, whether you're bird or human, so it is better to try and get her to eat a variety veggies than to add vitamins just to the water. Trust me, I understand how hard it can be! After two months, my RB2 hates eating anything BUT pellets, no matter what variety of veggies I offer, but I keep offering and she eventually eats a little more and a little more.
In the morning, offer what you want your bird to be eating, whether that is veggies or pellets or your seed mix and don't offer the millet until a couple of hours before bedtime. This will encourage them to eat what they have been refusing to eat. I would still encourage you to look into pellets and fresh veggies, not only for the vitamins but because you don't want your sweet fid getting overweight, but that is a personal choice in the end. However, I agree that sunflower seeds are pretty much useless for health reasons and should be saved for treats, so her not liking them doesn't really matter. You can find something else to use as a treat. (Possibly millet if you end up switching from seed mix?)