After Jan. 10, when the Securities and Exchange Commission approved the first bitcoin exchange-traded investment products, the biggest investment firms jumped into the pool with both feet, jostling one another to offer their clients, big or small, access to bitcoin funds. All, that is, except the second-biggest private investment management fund on the planet, Vanguard Group. The firm has made clear, most recently in a Jan. 24 message to its clients, that it has no plans to offer a bitcoin exchange-traded fund (ETF) or any other cryptocurrency-related products. Nor will it allow any such products from other firms to be...