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 healing uneventfully post-op, and given the reasonable blood supply inherent in a V-Y flap (although probably not as good as some other areas of the body, e.g., the face), I would be willing to irradiate the flap in continuity with the rest of the operative bed and areas at risk. I would likely compromise a bit on dose, however. I would probably stop large field irradiation at 45 Gy/27 fractions (170 cGy per fraction) before continuing with a boost...

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.