If your vested RSU's are handled by your employer and included in your W-2 as income then the employer is required to withhold taxes on those RSUs. At each vesting you should be getting a stub (like a payment stub) that breaks out all the tax withholding amounts for the amount of vested RSUs. The shares you receive as a result of the vesting will appear in your brokerage account and the cost basis of those share would be equal to the income that was reported on the stub (plus any payment you made for those shares). If you sell those shares later, that will be the cost basis used for calculating a capital gain or loss. BOTTOMLINE: Unless you sell your shares after vesting and have a capital gain, there should be no estimated tax due as your employer is already withholding taxes on your vested RSUs. Employers are legally required to withhold taxes on vested RSUs. The only exception is an 83(b) election, which probably doesn't apply in your situation.
