Drain line / vent question for new sink renovation

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Los Angeles
I'm doing a renovation and I'm installing a new sink. For some reason I can't wrap my head around how to properly vent the drain. I ripped open the wall behind the toilet and found two pipes, one I'm assuming is the toilet drain and the other, the vent.

Which pipe do I drain the sink into?

A few details: this is on the second floor of a two story building - there is nothing else draining above this toilet. These are the only two drain lines I can access with a sanitary tee (I'm also replacing the toilet). The sink will be going on the other side of the wall to the right. The sink drain will run about four feet (I'm doing the 1' to 1/4" drop as it travels). I'm assuming I'll have to add a AAV by the sink, 4" above the trap arm.

Is there anything else I'm not thinking of for the drain?

Hey, wait a minute.

This is awkward, but...

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