Hello,
We have the same concerns!!!
I think it is standard procedure to tell exchanges that they have no availability in the Cascades and Lakes. We have been told that already, and our trip is not until the first week of May next year.

They probably do keep these units available for actual Vistana Owners. But, I have also heard of time-share exchanges who are placed in Cascades or Lakes. Also, while the other sections are older. I understand that there are units that have been renovated and that are nice.
What I have been told specificly, by Starwood (The company that runs the Sheraton resorts), is that, while they do not have NON-Smoking units, they may be able to provide you with a unit with heavy duty filters. I have heard this from others reviews as well. I think if you make a big, but very nice and polite, point that you need a non smoking room, they may even place a special system in the unit prior to your arrival. They may also be able to do a more thorough cleaning job.
Also, be aware that the bed-covers are not changed every time like the sheets are. Perhaps you could request that they bring you some freshly laundered bedcovers. I think that would really help with any of that lingering smoke odor. You would at least want your bed to smell fresh.
Perhaps you could call personally, or contact diamond-lane again online, and make these specific requests.
Keep your fingers crossed... that is what I am doing!
Good luck!