The grant itself has nothing to do with the valuation. Each vesting sets the price for the gross number of shares received in that vesting. So each vesting is a lot and each lot has a per share basis that's the same as the per share "fair market value" used by the employer to calculate the compensation that's going to be reported for that lot.
Broadly speaking your basis is the same as that stock's "price" on the day of vesting, but of course a stock's price varies during any given trading day. Your employer picks a price to use - it might be the same price you sold the stock at, or it might not be - so the "most accurate" basis number to use is the same one your employer used.
If you ever end up holding some of these stocks for many years and lose all your paperwork on the vesting, but remember the vesting date, looking up the stock's closing price on that date would be "close enough."
