Can I vent off the top of a 90 turn down from upper floor?
Under IPC, not the way you show it. Don't know what code you are under, but they are often similar, while differing in details.
You could have a vent in that location, but there are two immediate problems I see with how you have drawn it.
Vent must be &quot;vertical&quot; (actual definition of &quot;vertical&quot; for this application is 45 degrees or more upward) to 6&quot; above the flood rim of the highest fixture on the floor served - you appear to show it travlling horizontally well below that level. It also cannot combine with any other vents below that level, so if the existing vent passes out through the roof below that level you'll need a new vent out the roof.
Dry vent Tee (or combo Wye) connection to dry vent stack is upside down. For a dry vent connection the bend goes upwards, and the entire vent is sloped to drain back to the fixture drains it serves.