maybe I am not following....... if you have 1000 RSU vest at a time when the stock sells for $20 per share, you are going to pay tax on the whole $20,000. Let's further say 200 of the RSU's are sold to pay federal, state, social security and medicare taxes, leaving you with 800 shares (with a cost basis of the same $20 per share).
later, you sell those 800 shares at a time when the stock trades at $25 per share. You would have $4000 of capital gains (800 * $5 per share)
does that help?
