Tie Into Vent Line for Lavatory Drain

As a followup to my post below, I have a shower floor drain that has a vent running vertically (up into the attic) in an adjacent wall. On the opposite side of that wall, I have a lavatory that is currently draining via thin walled pipe to the trap of another lav in a double lav configuration. I am wondering if it would be possible to have the lav drain directly into this nearby vent line (wet venting?). The shower drain is 2" and the vent is 1 1/2". The lav is less than two feet from the vent line.

You can't wet vent a fixture on the second floor over fixtures on the first floor.

Clarification: this is a ranch. Shower/drain/vent are all on main floor, as well as lav (which is on other side of wall where vent line is running). Vent is 1 1/2" and shower floor drain is 2". I am wondering if this vent line can serve as the drain (as well as vent) for this lav, without compromising its original function as simply a vent for the floor drain? If so, is there any requirement regarding height above the floor drain where I am allowed to tie into it (or any other requirements for that matter ). I assume I would use a sanitary tee fitting for this connection. Thanks

The wet portion is normally 2" pipe.
Fittings need to be waste fittings.
No santees on their back, only vertical.

drain

Again, you are describing a way to do it, but whether the way you actually do it is right or not is something else.