The 2" vent for the downstairs bath remains a vent. If you frame a wall in front, you can run from the left pipe over to the right of the vent. That line will be closer to the floor. Use a long turn 90 and a santee for the second lav, and continuing up, 90 back over and connect to the vent, which I'm guessing was installed at 42" from the floor.
You should have 2" if you are using two lavs, but you don't have that. You will have to make do with the 1.5" that you have.
You also have the option of opening the floor and running the line over to the second lav in the flooring system.
Or you can open the floor and put the vent to the right of the new lav. Then you wouldn't need to build out the wall.
See how fun this is? Lot's of ways to do this.