Your actual per-share cost basis in the RSU's is the compensation created by the vesting (which is reported on your W-2) divided by the gross number of shares you received - not the net shares you received after shares were sold or withheld for taxes.
Yes, if you incur a capital loss when you sell the shares, you may use it to offset your capital gains.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
