bathroom vanity drain to vent stack question...
I bought a house built in the early 50's for a fixer-upper for myself. The Vent stack is 3" PVC running straight vertical from the crawlspace which is connected to cast iron. I can not relocate the stack under the floor of the bathroom as it is next to cement foundations on 3 sides. The kitchen sink, the toilet, tub, and bathroom vanity all drain into this stack under the floor directly under the bathroom. The issue is that the stack coming up into the bathroom is 18" in from the one wall. I wish to try and reclaim some of that space by a couple 45's in the stack just above the floor in the bathroom, back to the wall, then up 6' into a couple more 45's to put me back at my existing roof exit for the stack. The problem is the vanity drains into the stack 1' above the floor. I would "like" to start the 45 in the stack as close to the floor as i could (if I can) so I can put the vanity over top to reclaim the space, but can I drain into the stack if the 3" 'y' connector is in the 45 degree part of the stack, not the vertical part?
I have attached a simple paint drawing illustrating what I mean, the red circled area is what's in question because it will not drain into the stack at vertical.
The second image is a top down of the current bathroom layout, I'd like to move the tub and Vanity back towards the wall about 8" to gain some more space.
If I can not do this then unfortunately because of all the obstacles in the way in the crawlspace I will have to leave the existing wall that was built to house the vent stack in place and lose out on some extra space.
Thanks for any input and/or recommendations!