Sorry to hear that you are having to deal with a flare so close to your vacation! That really stinks!
I can't offer alot of advice on Pred since this is only my second time around with it. I have been on 40 mgs for about a week now, and I have started to notice small improvements. I am tapering down over the course of eight weeks. I don't have diarrhea, so I can't testify to how it will improve that aspect for you. I did have a crampy pain in my lower left side that I haven't felt since about my fourth day on the Pred. My main problem has been blood/mucous and it has helped, slowy but surely.
I think maybe the reason I have noticed a "WOW" kind of improvement is because my symptoms are horrible, i.e., major pain and diarrhea.
My doc told me that with an eight week course, I would hopefully avoid heavy side-effects.
If I were you, I would take the Pred. (1) I would be really tired of going to the bathroom that much - I feel so sorry for you guys who have diarrhea. (2) You could eat!(3) You would have energy for your vacation. (4) Maybe the cramps would lessen and you could ENJOY your vacation - which you TOTALLY deserve to do!
Whatever you decide, I am sure it will be the right decision for you. Nobody knows your body like you!
Have you tried Immodium for the diarrhea?
40 mgs of Pred