Update: I took photos of the vent pipes but they didn't turn out well because of vines growing. I located a total of three vent pipes. Two were small ones (I'll have to try to measure to see just how small) and one looked to be about 4 inches.
The first smaller one was coming out from under the kitchen sink to the east wall directly under the window. It made a 90 degree turn to go around the window and then went straight up to the roof where it just stopped (nearly touching the soffit).
Here are crappy photos of the vent pipe near the kitchen sink.
Here is a shot of the roof vent pipe (also a bad quality pic)
Here is the largest pipe and how it ends (it comes out from the west side of the house just north of the window to the northwest bedroom)
Another shot of the larger vent pipe.
Here is a rough floor plan I sketched of the house. Blue marks the bathtubs, toilets, and sinks. There are two bathroom sinks and a kitchen sink as well as three toilets and two bath tubs. I do not know where or if the useless bathroom's sink and shower are tied in or not.
I somehow suspect the house is not vented properly. At least we have vents now. When we came back from overseas the tenants had ripped out all of the vents and screwed up our water lines.
Out of curiosity, what would be the proper sizes, order, and distances for these vent pipes? Is it normal to have so few and of such small size?