I'm certain the 3" pipe on the left is the toilet drain (it is about 12" form the wall and has a rounded cap on it). I'm guessing the shorter 2" capped pipe on the right, up against the wall, is the vanity drain. Then there is a 3.5" pipe on the bottom right that has a plug that looks like it's screwed in, but is actually not screwed in (it pops straight out). From what I understand, this is definitely not for the shower and is most likely a cleanout. Looking down it, I can see the water go by as my wife flushes the toilet up stairs. This seems like an odd place to put a cleanout, given the location of, what I believe is, the vanity drain. Also, I think the pipe that goes to the ceiling in the center of the picture is the vent stack. There appear to be larger pipes that come down the wall further to the left that I believe is the main drain that runs under this set of pipes.
Here's a photo (click for full size):
So, 3 questions:
Is that 2" pipe most likely for the vanity?
If so, does this mean I'll be tying in to the vertical pipe for a dry vent, although there is no roughed in horizontal pipe pointing at the 2" drain?
Is that horizontal pipe pointing to the left a vent pointing to where the shower is intended to be located (as in that is the dry vent for the shower)?