the shares that were delivered to you on that date have a cost basis equal to the price of the stock on that date
Until you sell the shares that remain, there is no taxable event. You will receive a 1099-B when you do that.
ps - the shares that were sold to cover the taxes have no tax impact to you. Look at your W-2; the value of ALL the shares that vested is taxable income to you and should be part of your wages.
