well when you take off the door card, right underneath should be a big plastic sheet sealed to the door with some silicone adhesive. other than that, you have the seal at the top of the door body that tries to prevent huge amounts of water from getting it (this is mostly when the window is up and pushing against it), but you will always get some water in there. come to think of it, water does drip on the motor and i've seen a few posts about rusty motors because of it. for your speakers, just get some speaker baffles to protect them.
no, there is no problem with vapor barrier. i just dont have it and actually dont know if its of any use.
inside of my doors was intensely filling with water for about 5min but nothing got in the cabin.
But the motor was swimming. I dont see any other protection for it other than the window gusset.
ill try to slowly pour a glass of water on the window and see if its still that bad.
and yes, now im sure i need those speaker baffles.