So we are gutting the bathroom due to a leaking tub and toilet. The only thing that didn't link was the sink, amazingly. We had to sister studs, replace underlayment, subfloor, etc etc... the only problem is we want to put new fixtures in and I want everything to be code. I have been debating on using an AAV (studor vent) to rectify the lack of venting for the tub and sink but I am wanting to make sure there are no other glaring safety concerns on the design of the cast iron vent stack. It seems to me that having the water rush down the toilet pipe past both the sink and tub would suck the water right out of the traps since the sink/tub inlet are between the toilet and vent riser. We have never had a problem before and we never notice any smells but I know it can't be right. As for the AAV for the sink I want to make sure I can put it in the vanity 4" above the sani-tee. I am pretty sure it is 4" since it is not the MAIN vent but again, not a plumber, want to check. I have taken some high resolution photos and used paint to show proposed changes. Thanks so much in advance for any assistance you may provide.

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if you put the studor vent up under the sink cabinet, that will wet vent the tub as well. technically the sink drain should be 2 inch if it is going to act as a wet vent for the tub but i would not worry about it.
although, what i would do would be to cut the galvanized nipple that is betwen the Y and the stack. cut it right close to the Y , clean it out really well and slide a stainless sheilded coupling onto it, and PVC pipe from there. roll the branch of the Y up if possible and put a clean out tee in that pipe.
last, the inlet for the trap should be more towards the end of the tub. the inlet sets under the overflow. the shoe sets under the drain hole. a tee connects them together.

Thanks for the reply. Just to verify I understand, you think I should get rid of the cast Y adapter, but instead of trying to rotate it off the galvanized nipple just cut it? Is unscrewing the Y a bad move? A danger of damaging the cast? You are also saying to move the Y closer to the tub so the tub drain has a straighter shot to the wet vent? Makes sense, if that's the case. One more question: I need to move the hole for the drain from the floor to the wall for the new vanity (it has an open leg design) would increasing that drain size for the sink to 2" as you suggest increase the chances of clogs when it is reduced to 1-1/2? I assume there are special "clogproof" sweep Ys for this but I wanted to check. Leaving it at 1-1/2" would be fine too if you do truly believe it won't make any difference. I have to notch out a non-load bearing bottom plate and the small the better. Thanks so much for your time

it sounds like we are talking the same language here.
you can not increase that drain to 2 inch because the nipple at the stack is only 1 1/2. if the nipple will unscrew from the stack, you can remove it and screw in a male adapter. other wise just cut it leaving a good inch and use a stainless banded coupling.
the notch you need in the 2 x 4 plate can be notched at a 45 deg with a short piece of plastic laying in it. that pipe will be at a 45 deg slope with another 45 on it to bring it to vertical.
put the stub out at about 18 inches above the floor.