Would you include the entire op bed (including flap) within the radiation field in a patient requiring a V-Y advancement flap for closure following a radical vulvectomy?  

It depends on one's assessment of the risk of recurrence in the region of the flap. It sounds like the positive margin is pretty significant and situated at the vaginal introitus. Most likely, the area of the flap is at risk, but this assessment should be individualized. Assuming that the flap is he...

Yes, generally, it includes the entire operative bed.

Yes, I would include a full flap in the microscopic dose field prior to the boost to a positive margin, provided the flap is healthy and well vascularized. Prolonged or poor healing may compromise full flap coverage to limit the risk of dehiscence.