Yes, you need to provide the cost basis, just because the IRS says so. They don't do anything with it. It's mostly for statistical purposes (they want to know how much this tax saving gimmick is giving away). If you can't find the basis, a guess is close enough.
There is no problem in TurboTax. You are expected to know your basis. You problem is with Vanguard, not making it easy for you to find.
