Final answer:
The nurse should advise the postpartum client to apply ice packs to the perineum to reduce swelling and alleviate pain after a vaginal delivery.
Step-by-step explanation:
To promote comfort after a vaginal delivery when a postpartum client complains of perineal discomfort when sitting, the nurse should provide the following instruction: 'Apply ice packs to the perineum.' The ice packs help to reduce swelling and numb the area, providing relief from pain. While other measures such as performing Kegel exercises, using a donut-shaped cushion, and administering analgesic medication as prescribed can also be helpful, the direct application of cold is typically advised in the immediate postpartum period for alleviating perineal pain.