There are several ways to install a bow vent.
My code book allows 1-1/2" drains with 1-1/4" vents BUT, I would suggest going with 2" all around.
The drain is connected to a vertical stack below the vent.
There cannot be anything draining above the vent in that stack, both are connected as if they are both drains...use the assumption that the vent will be a drain.
If any of this is unclear, I'd suggest having it done by a plumber, once that slab is repoured - it's all over but the cryin'.