The vent must come off the top of the fitting.
I would install a santee vertically, with the middle portion going to the tub p-trap
The top would be the vent.
Below that, you could then offset with 45's
I am planning to run part of a 2" bathtub drain down at 45-deg for a short (2 ft) distance. As it turns out the place we'd like to vent the fixture is along the 45-deg drop. So I am wondering if there'd be anything wrong with using a 2" Wye fitting such that the inflow-discharge segment is aligned with the drain at 45-deg to horizontal and the branch at 90-deg to horizontal was used for the vent?
Also, is it generally better to drop down 2-3 ft with 90-deg bends than with 45-deg bends?
Is there anything wrong with it? Definitely. In fact, in that application, there is no way to use a "Y" properly. In fact, once you install the trap, the line CANNOT turn down at ANY angle, until you connect the vent to the line. After than you can drop at any angle you want to.
Terry & HJ thanks for your replies. Running from the trap I am going 2ft then making a 90-deg turn and I guess because the run was getting long from the trap I was forgetting I am in fact still in the trap arm and so of course I cannot vent below the weir level... and in fact I can do as Terry suggests which is to use the sani-T just like the termination of any trap arm.
Well anyway, that's why I asked... it didn't feel right but I couldn't say why.