Because this activity happens through the employer to the broker, you need to use both references. The vesting of the stock is the date the value is considered taxable, which is why the value is part of the W2. To meet withholding requirements, some of that stock is sold by the broker, which is why you see a 1099-B. Your taxpayer might or might not have sold all their stock, so you should document what they still have ownership of and what is that basis, for when they sell later.