Google "closed vagina". You will find a lot of info. I just noticed my almost 3yr old has it b/c I noticed that she looked different from my 6 month old.
If it is closed entirely, it will need to be opened at least slightly in order for her to have periods. However, I'd have the doctor check her, to be certain it is entirely closed, before you worry.
It is ok nothing to be worried about when wiping the baby vagina clean it out well wil a baby wipe do not use so much disatin for the diaper cream and convert to cloth diapers during the time either at night or during the day when you are with her. It is probably not that it closed up it may look like it because the skin is pinched together it is probably that she is allergic to the diapers or the skin wants to breathe.
At two and a half i would not worry about diapers so much if she is a baby then that would be different. I would contact the dr my thinking is that the skin wants to breathe i would let her go with out a diaper for a hour or two a day if you feel she will pee or poop i would try a cloth diaper something that will let her genitals breathe well like all organs skin is the organ that needs to cool its self and able to let the air cleanse it self.
It is probably not a big worry i would call the dr they will tell you the same not to worry unless it is completely closed and no urine is coming out the worse case is a minor cut to open the vagina and stich but that is not the case as i read it. It happends to babies and kids around that age so i would think it is something you tell your dr but not to over react about !