My understanding is that you are now carrying a capital loss, but you can't claim it against your income tax, you have to wait until you have your next capital event as this loss is ringfenced. See example below:
value at vesting = 100 (this is the taxable value for future sale)
income tax paid = 52 (income tax, prsi, usc)
sales value received = 99 (as this is below 100, no further Capital gains tax CGT is due now)
net cash gain = 47 (99-52), with a loss of 1 (100-99) carried forward to your next capital transaction (shares or asset sale for example).