I am a diabetic as well and I found that when I went through rush there was always sometyhing I could eat or drink, like water or veggies or fruit. (I'm also a bad diabetic so I would eat the chocolate too
)
Currently we have a sister who has PKU and we try our best to find foods that she can eat. We also have several sister who are vegitarians or vegan and we usually try to have something like veggies or fruit available.
You'll probably find that the parties aren't all that long and you'll be too busy too eat. But you're not the only one who is a vegan and you'll find that there will be members of the houses that are vegan/vegitarian. It's really nothing to worry about.