The warm bath seems to have helped some, as it came farther out and the vent area can be seen now.
Just like @Eggcessive already mentioned, it looks like a pus accumulation mixed with faecal matter etc.
If you cannot get it out right away, just repeat the soaking in warm epsom bath as often as...
Here is what I would do:
Soak her well in a tub with warm epsom salt water for some 10-15 minutes and try to carefully get the stuff off her vent to allow for a better assessment of the situation.
Does she eat and drink normally?
Does she seem in any pain and can she walk normally?
When did...