Vanguard, one of the largest asset managers in the world, will not be allowing customers to buy the newly approved bitcoin ETFs.An attempt to invest in BlackRock's iShares Bitcoin Trust (IBIT) or Grayscale Bitcoin Trust (GBTC) via a Vanguard retirement brokerage account generated a "trade cannot be completed" warning.Attempts to buy IBIT and GBTC through Vanguard were blocked. (Vanguard)"Buy orders are not currently accepted for this security," the message said. "Securities may be unavailable fo...