Because Social Security and Medicare taxes are due when the this deferred compensation is no longer subject to forfeit, the contributions should appear in box 3 (subject to the Social Security wage limit) and box 5 of the W-2 at that time. If prior year contributions are vesting, they and their attributable earnings also need to appear in box 11. See examples 1 through 3 in the Nonqualified Deferred Compensation Reporting Examples Chart in the instructions for Form W-2: https://www.irs.gov/pub/irs-pdf/iw2w3.pdf
