Notwithstanding the subject line, I wouldn't buy vitamins at the 99 Cent store, 'cause you never know what's in them. I would hazard a guess that the women's vitamin that your niece is now taking has 100% of her daily iron requirements, and she might not have been getting enough, thus the change in her energy levels. That's just a guess, though.
I think the key for any vitamin is that you trust the source--I trust Wal-Mart, because I know that most of their store brand products are made on the same production line as the brand they are costcutting - Wal-Mart's just cut a deal to get them to the consumer more cheaply.
My own opinion (and it's probably worth what you paid for it...
) is that, if the active ingredients are the same, the vitamin is the same. The only place where I have to be careful is the coatings--the dark red coating on some multivitamins makes me ill (and it did pre-op as well), so I have to take the ones with the peach-colored coatings. Other than that, I just try to get some balance. My last labs were done in March, and were spot on.
Here's my daily vitamin regimen, and the reason why I take 'em. Please note, I've not been able to take my 1500 mg of supplemental calcium since my gallbladder surgery, as it makes me hurt something awful (don't know why). Still trying to introduce it slowly, but since I get 90 percent of my protein from lowfat dairy, I'm not sweating it too much.
Morning
Prenatal 19, 1 daily, as multivitamin, iron supplement (by prescription)
Beta Carotene, 25K IU, for Vitamin A levels
B-12 sublingual, 500 mcg, for B-12 levels (and because B-12 in multivitamins is not readily absorbed)
Flaxseed Oil, 1000 mcg, to maintain regularity
Fish Oil - Omega 3, 1000 mcg, family heart disease history
Evening
Flaxseed Oil, 1000 mcg
Fish Oil - Omega 3, 1000 mcg
Centrum Multi-Vitamin