Mark Vadon liked buying a diamond on the Internet so much, it changed his life. “I was out shopping for an engagement ring a year ago,” he recalls. “I went on the Internet trying to get information and ran across Internetdiamonds.com. I got a fantastic ring, and it was just a very good customer experience, and I was intrigued with the model.” Vadon, who has a financial background, was so intrigued he became the Web site's CEO. Venture capitalists were similarly interested; seeing a niche that hadn't been scratched, they gave the company $40 million last year. With its coffers now full, the Web site is changing its name to www.bluenile.com and wants to be the first nationally known jewelry-selling Web site. Now if this were your typical Internet success story, the company would go public, its founders would become millionaires, and everyone would ride a new sports car into the s