Some of the shares had to be sold to pay the taxes on them ... that would have been reported on your W-2 and a 1099-B from a broker. The rest would have been "banked" for selling later. The IRS got a copy of the 1099-B showing the sale and you had to report it on your return ... usually the sale would be a loss in the amount of the brokers transaction fee since your basis in the stock was what was paid thru the payroll system. Ask your employer for the 1099-B or get the brokers name so you can get one from them.
