this is not a 'neighbor' problem and no letters should be sent to the neighbor -- it is a landlord/ building problem
odors from one apartment should not be leaking this much into another -- there is clearly something wrong with the ventilation system
the OP might approach the neighbor with the problem 'with the ventilation' and its impact on her -- and do some problem solving with the neighbor -- perhaps if she didn't smoke in the bathroom or kept that door shut, it would cut down on the leak -- or they could explore ways to track down just how the smoke is leaking through -- and try to do what it takes to prevent this
this should all be approached as 'a problem with the building' and with the attitude that 'of course she has the right to smoke in her own apartment' --
with or without information about where the leak is -- approach the landlord about a problem with the ventilation system that needs to be fixed. -- and be persistent -- about following up
Jaywalker, I think you've hit the nail on the head. This really is a ventilation problem, not a problem with smoking. It's not like the OP can actually ask the other tenant to stop smoking... If she knew the other tenant at all, I might recommend talking to her, and seeing if closing a door in the smokers apt. can help the situation, but since she doesnt know the other tenant, going to the landlord is not going over her head, because it's not the other tenant's responsibility to fix the problem (ventilation). It's the landlord's. So, writing to the landlord about 'the ventilation problem in my bathroom' is the appropriate first step.
1. odors from other apartments are making their way into your apartment so much so that your clothes are smelling like them.
2. your vent is rusted shut.
these both warrant a letter/phone call to the landlord.
The landlord may be able to reduce the problem with better ventilation, but the nature of living in apartments is that sometimes, odors/noises from other tenants are unaviodable (yes, in your case it is an extreme, but small amounts of both are to be expected in apartment living). I would definitely invest in an odor-eater/air purifier and try to remedy the situation on my own IN ADDITION TO what the landlord fixes.