I have the exact same issue. I contacted Vanguard, and they essentially said that they don't have sophisticated enough accounting available to customer service to answer the question. The fund in question in my case has a percentage of U.S. obligations assets in the fund, and I would be tempted to use that percentage of the gains indicated in the 1099 B for that fund. I know that would not be accurate, but it would likely be closer than zero. (And having spent hours on this at this point, I think I have earned any benefit.)
I wish all of us good luck and better coordination between the state of Maryland and Vanguard in the future.
