The only answer I see is telling me how to figure out my acquisition date and basis.  I know these items.  My question is how to enter them in TurboTax, since Vanguard's 1099-B has blank boxes for these items.  Should I enter my known data in TurboTax as if it was provided in Vanguard's 1099-B?

The Basis of the Stock acquired from a decedent 

1.is Fair Market Value on the date of death of the decedent    OR

2.if the personal representative of the Estate elects, 6 months after date of death.  

Whichever date you choose will also be your acquisition date.   

Note you get a steped -up basis in inherited property.  So your basis (cost) in the stock needs to be determined as the FMV on either of the two dates above.

If you broker can not help you determine historic FMV, research on line under the actual stock name or corporate name can usually turn up this information.