Vitacost sells the majority of their products for far less than you'll find them elsewhere because they are able to buy in such huge quantities. They operate on selling mass quantities so while their profit per item might seem minimal, when they sell in such large amounts, they do quite well. Look at Sam's Club, Costco's, etc who sell many name brands for so much less than these brands sell elsewhere, including their own sales sites or stores but they too operate on mass quantity sales.
Selling mass quantities enables them to negotiate better prices with the manufacturers which they then pass on to the customers with a real reduced profit per sale. Yet multiple those sales numbers by the thousands and they secure pretty decent profits themselves. Every penny they get in discounted prices makes a big difference.
A site like Vitacost couldn't risk selling other than top quality merchandise, especially considering they are in the nutrition, supplement and vitamin business. It would shut them down very quickly if customers thought they were receiving anything other than first rate quality.