This assumes you already know the size of the gap. If not, our RSU withholding shortfall calculator works it out from the vest, your salary and your filing status.
Three ways to close it
Once you know the size of the gap, there are three reasonable responses, and they differ less in the total you pay than in when you pay it and what happens if you are late.
Raise withholding on your remaining paychecks
Adjusting your withholding spreads the amount over what is left of the year. Withholding is treated as paid evenly across the year regardless of when it actually happened, which is a genuine advantage: it can repair an earlier underpayment in a way a late lump sum cannot.
Make a quarterly estimated payment
An estimated payment is dated when you make it. That is the trade-off against extra withholding. It is faster to arrange, and available if you have no remaining paychecks, but it does not retroactively cover an earlier quarter.
Set the money aside and pay at filing
Perfectly reasonable if the gap is small enough that no penalty arises, and the worst of the three if it is not. The risk is not the tax, which you owe either way; it is the penalty and the possibility of having spent the money.
The bit that decides it
The deciding factor is usually the underpayment penalty: whether the shortfall is large enough to expose you to one. That depends on your total payments across the whole year, measured against safe-harbour thresholds rather than against any single vest. Which is why a calculator looking at one vest cannot answer it.
If your vests are large or repeat through the year, this is the point at which the answer stops being arithmetic and starts depending on your whole return. That is a good moment to ask someone who can see all of it.
Start with the size of the gap
The RSU withholding shortfall calculator gives you the number these options are about.