A difficult case no matter what you do--obviously appears to be a colovesical fistula, requiring simple excision--actually more likely to be from diverticulitis than Crohn's but either is possible. I would resect and anastomose--give Vitamin A perioperatively to counteract the wound-healing detriment of steroids. Just recognize, along with the patient , the high risk--you are not solving much by doing colostomy--what do tyou think of this patient's chances of coming thru another operation without complication when you close the colostomy? What is the benefit of subjecting her to two rather than one operation? she will still have Crohn's disease and be on steroids when you bring her back to close the ostomy, so what are you accomplishing? Obviously your abdominal fascia is also at higher than normal risk of dehiscence--would that keep you from closing the fascia, and just packing open to come back another day? Think about it--there is really no reason to avoid an anastomosis, as it will not really increase the risks over any other approach--rather, do the right thing, and do your best to minimize risks, including trying to keep operating time, and number of procedures, to a minimum.