Might be a rectal prolapse? If you scroll down the page to find the pic, then look at mouse "C. Grade 2" and "D. Grade 3", they both have rectal prolapses:
http://ajprenal.physiology.org/content/295/2/F545.full
(Note: the article is about something different entirely, but the pics make it pretty clear what a rectal prolapse looks like.)
When I was first starting up my mouse colony, I had several mice get rectal prolapses, so I researched it at that time. If I recall, the 2 most common causes of this are some sort of intestinal parasite or some sort of intestinal infection that causes diarrhea. In my mice, it seemed to be the diarrhea infection (although I actually saw very little diarrhea), and I treated the colony with tetracycline.
As far as the mouse that actually has the prolapse, the outlook is not good. Again, I'm remembering stuff I researched a few years ago, but I'm pretty sure it said that even if you can get the prolapse to go back in, it is likely to recur. So I'd suggest just feeding off the mouse with the prolapse.
Here is a link to the thread I had about it. It has some more detailed information and a few more useful links:
http://ball-pythons.net/forums/showt...ice-prolapsing
EDIT: I see since I started writing this reply that you've decided it is a prolapsed vagina, in which case my reply is kinda irrelevant. I'll leave it up in case you change your mind about your diagnosis or maybe it will help someone else.
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