That $15/purchase is probably waaaay more than the equivalent amount of Vanguard fees, unless you're buying thousands of dollars in stocks at a time. And those fees add up to a significant amount over the lifetime of your portfolio.
Speaking of your portfolio, if you're investing in "a few" companies, your volatility and risk are going to be enormous. Especially if you're not balancing your portfolio between industries, company sizes, and less risky investments. Automatic rebalancing is a fantastic way to obey "buy low, sell high", as you intrinsically buy more of cheaper instruments and less of expensive ones to maintain that balancing.