What's the easiest/cheapest way to vent existing plumbing?

We own a single family house which was converted to a duplex, (I believe by Larry, Moe, & Curly) & decided to change it back after 4 deadbeat tenants let us down. After repairing pipes that where leaking waste water into the space between the inside & outside wall, biohazard clean-up, & purchasing materials to replace the walls & rotting floor, my friend noticed that the 2nd floor bath wasn't vented with the rest of the plumbing. I'm sure it would be easier to vent before we rebuild the 1st floor ceiling & walls. I'd like to have walls up & insulated before the snow flies.(It gets cooold around here) I'm very smart, but i'm not a plumber, so a simple diagram would be greatly appreciated.

The second floor can vent through the roof on it's own, or it can be tied back in.

Thank you very much. Diagram was very helpful.

The average consumer is NOT in a position do decide anything about venting. Call a plumber to assess the situation and see if ANYTHING has to be done. It would be very unusual for an upper floor fixture to NOT be vented. A picture of what YOU have would also be helpful.