Again, thank you all for the comments and advice.
I really want to save money by doing this myself as we just bought this new house and there are other things to spend money on, like a roof, which I won't dream of doing.
Sorry for that lousy pic. I had taken it with iPhone just as a reference when I went to store.
I hope these are better.
First, here is the entire end of the bathroom. Tub will go up against wall and to the left. That's why HVAC vent was moved up.
Here is a close-up of the cast iron toilet drain, which after considering it, we decided NOT to remove it. We will just find a toilet that is narrow enough to sit next to the tub.
Finally here is the area where the tub drain will be. The reason the "on the floor" configuration was chosen was because the drain on that side of the room, which loops around the room counter-clockwise to the toilet drain is just barely under the sub-floor. The tub drain will be at the right side of where the sub-floor has been removed.
I already cut out the previous sink drain and T Wye.
But as it is now obvious to me that using the existing copper DWV drain piping is just not worth it and that switching to PVC is a better idea, we will instead try something else.
The tub drain will be used in a "normal" configuration, with pTrap connected to it (as Mr David's image shows). Will still utilize the existing vent, but rather than connect to that 2" copper drain, will route it back directly to the toilet drain and utilize the existing 1 1/2" inlet, which was used before for the shower.
I'm assuming since the drain for the tub comes with 1.5" pipe, I can use 1.5 inch drain back to that toilet thing (which is 1.5") along with a 2" vent.
I do have to figure out how to attach to the bottom of that vent in some sort of u-turn thing with drain going back in the other direction.
Is that a bit more clear?