How do I enter a 1099-B for an EOP / Equity Ownership Plan when the taxpayer's income taxes were collected by the employer on the vesting date?

"and the related taxes withheld were in the taxpayer's W-2."

This never happens. Employers are not income tax authorities.

Your taxpayer, as with all employees, provides a W-4 to the employer, so that the employer can use the payroll tax tables to provide an estimated amount to take from the employee pay, as a prepayment to the Fed (and perhaps, the State) against your taxpayer's likely taxes owed when you prepare that taxpayer's tax filing.

It's never Tax. It's called Withholding. It's a Prepayment against taxes.

Any award of significant value is taxable. There are withholding requirements, such as flat rates on bonuses, amounts held out of RMD, etc. None of these are Tax. They are the same as an estimated payment the taxpayer can and would make on their own, if they opt out of all withholding.

Oh: Vesting date is the Basis date and value. 1099-B means something was sold. The sales sometimes are of holdings that vested across many years. That's why you need the W2 from those years. And sometimes an employer will sell (or have a broker sell) some vested shares to cover withholding, when the value of this bonus exceeds the ability to withhold from the regular paycheck amount.