will the Federal tax on the vested RSUs remain at 25% or they can/should be adjusted to the final tax bracket?

They will remain at 25%. Bonuses and other non-recurring income will always be taxed at a single rate, because there's no way to predict what your actual tax bracket will be. With periodic income, they can extrapolate that out to your annual income and be more specific with the amount that is withheld.

If your final tax bracket is less than 25%, it means more was withheld than was necessary, and you will get the extra amount back either in a larger refund or a smaller tax bill (depending on your other withholdings).

I see that in 1040, the capital gains/loss is only a relative number with respect to the cost basis. The amount reported in W-2 doesn't show up!

Correct. In addition to the tax withheld for the RSU (which is considered income) you also need to account for any gain or loss as capital gain/loss.

will the Federal tax on the vested RSUs remain at 25% or they can/should be adjusted to the final tax bracket?

They will remain at 25%. Taxes on bonuses and other non-recurring income will always be withheld at a single rate, because there's no way to predict what your actual tax bracket will be. With periodic income, they can extrapolate that out to your annual income and be more specific with the amount that is withheld.

If your final tax bracket is less than 25%, it means more was withheld than was necessary, and you will get the extra amount back either in a larger refund or a smaller tax bill (depending on your other withholdings).

I see that in 1040, the capital gains/loss is only a relative number with respect to the cost basis. The amount reported in W-2 doesn't show up!

Correct. In addition to the tax withheld for the RSU (which is considered income) you also need to account for any gain or loss as capital gain/loss.