adding a vanity

Hi all, trying to finish my basement and have the following question (forgive the crappy drawing). I have existing plumbing rough-ins for a shower, toilet, and a sink. I would like to add a sink and/or washer marked in maroon (bottom right). The the green upward arrows are vents for the existing...

I am remodeling one unit in a duplex. The tenants before have had issues with the toilet backing up into the shower. I want to add a vanity (only) behind the shower into the master bedroom but I don't want to make the issue worse. I have taken several pictures and spent many hours under house...