Thanks for the reply!
Yeah, I wondered about that vent.
As for moving the plywood a few inches, well that would be removing it. The rafters are 2x4 and are almost right on the header for the wall so I'm gonna just remove them and adjust the insulation so there is a clear path up to the high vent.
While checking again today I also found that the stack is completely open to the attic for about 2 feet from the attic floor to the ceiling on the high side
Looks like they didn't want to deal with the vent for the plumbing so they just stapled the vapor-barrier around it and when whoever put in the yellow insulation was at work they pushed so hard it opened up the barrier...
I guess I'll need to go through the bathroom wall to fix that and seal it.