I have ulcerative colitis and belong to a Crohn's and Colitis support group. Also last year I went to a CCFA patient education conference, and a female GI spoke on women's issues and I took careful notes. So here's the scoop:
With Crohn's patients, it all depends on where the disease is located. A woman with perianal disease such as inflammation or abcesses around the anus should not have a vaginal delivery, because that is basically guaranteeing a recto-vaginal fistula. The skin just won't hold up. Also a Crohn's patient with an existing recto-vaginal fistula, or recto-perineal fistula, should not have a vaginal delivery because it would make the fistula worse. Likewise, there are some types of adhesions (either from disease process or from prior intestinal surgery) which make it difficult for the uterus to contract properly. OTOH, a Crohn's patient with, say, esophageal and small bowel inflammation but no perianal involvement and no adhesions of the wrong kind can easily have a normal vaginal delivery (though sometimes they don't know they are in labor, being used to pain and pressure).
Ulcerative colitis patients can pretty much always have normal vaginal deliveries. Unlike Crohn's, UC does not increase miscarriage risk by much, so is not automatically considered a high-risk pregnancy the way Crohn's is.
Finally, with both ulcerative colitis and Crohn's patients, there can be a mechanical problem which would inhibit vaginal delivery. If there is damage to the rectal muscles, or if the rectum has been surgically removed, pushing can be pretty much impossible. I do know one woman who birthed vaginally who had an ileostomy and her colon and rectum had been removed, but she couldn't push. It required big-time forceps and vacuum. So people with no rectums or with rectal muscle damage usually prefer C-sections.
No one with inflammatory bowel disease should have an episiotomy because even ulcerative colitis increases the risk of episiotomies' becoming recto-vaginal fistulae (though unlike Crohn's disease, UC does not cause spontaneous r-v fistulae without childbirth).