First, make sure you have your fact situation figured out.
WERE shares sold "for taxes"? I assume the IRS letter says something along the lines that the income tax return is missing a sale reported on a 1099B? You said that no shares were sold and if that really and truly is the case then that IRS letter is just dead wrong and you have to respond to that.
If shares were sold, then was a 1099-B issued? As I said, if no 1099-B was issued then the IRS has no visibility to the sale so I'm guessing a 1099-B WAS issued. With your new understanding of how these things play out perhaps talk to the broker again and ask that specific question.
The compensation created by the vesting is (GROSS number of shares vesting) x (per share "fair market value" used by the employer). This figure is typically disclosed in Box 14 of the W-2 and "RSU" is usually included in the very brief description. The "per share" basis of the stock is the same as the per share fair market value the employer used. That's why a "same day" sale typically generates a small loss because the amount received from the sale is less brokers commissions and fees.
If the IRS has said it's not necessary to amend - although you may if you wish - a simple letter explaining the shares were sold the same day the RSUs vested and resulted in a small loss, maybe accompanied by a copy of the W-2 with the Box 14 amount and description highlighted, should put things to rest. I'm sure the IRS sees thousands and thousands of errors of this sort.
