My house is from the mid 50's and I noticed a leak down in the basement. I found the leak coming from the drain pipe that comes off the tub, but after further investigation, I discovered that the pipe is also tied into the vent stack/drain with two 90 T connections. I don't know why it was built this way but it seems that the upper level pipe is the drain for the sink in the bathroom and the kitchen and the lower pipe is the drain for the tub and there are two pipes connecting the two vertically. The pipe appears to be soil pipe and I was wondering if I am able to replace that with PVC because 1. I would have to get a piece made custom for that and 2. PVC just seems easier with the exception of the 3 joints I will have to do

You can convert that area to pvc , no problem. Just cut out the section you wish to repipe, with a soil pipe cutter or sawzall with a good metal blade , and use some fernco's to hook on to the old pipe and repipe with pvc..

If I use the size pipe should I attach it with a rubber joint or can I set it in the hub joint?