RSUs are taxed as income to you when they vest, and their value is included in the compensation reported in Box 1 of your W2. That income is subject to federal and employment tax (Social Security and Medicare) and any state and local tax. The income is subject to mandatory supplemental wage withholding.
Thus your taxable income in your scenario would be $400K.
When you sell your vested shares, you will have a capital gain or loss, based on their change in value since the vesting date.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
