Yes, your employer probably did withhold taxes when the stocks vested. If you believe that the 1099-B accurately reflects the cost basis of the stock, which is to say the fair market value on the vesting date, then you do not need to put in any information about vesting. Just treat it as a regular stock sale rather than an RSU sale by selecting Stock (non-employee) as the type of investment.
The vesting information is only needed if the cost basis and acquisition date reported by the broker does not accurately reflect the fair market value and date when the stock vested.
If the cost basis is not adjusted, you may be getting double-taxed for the compensation element, once in the year that your stock vested and it was added to your W-2, and once now that you are selling it.
