I have recently purchased a house and have run into a unique situation (at least it seems so) with the flooring. Some background: The house was built in 1952 and is 3/4 on slab 1/4 unfinished basement, located in central MA. The area I am concerned with is the slab area. There is an existing hardwood floor 2 1/4" wide oak 3/4" thick that has been covered over with glued and stapled 1/4' lauan plywood as an underlayment for carpeting the previous owner had installed. I initially figured I would remove the lauan and refinish the hardwood but after looking at it and pulling up a couple test sections it will take forever and be near impossible to property remove all the staples (1 of ever 5 break off). Even if I had the next 2 months free who knows how all the holes would stain? Next, I decided that I would pull the whole floor and nail the new floor to the sleepers, this is where things get interesting. I pulled a test section of the hardwood and there are no wooden sleepers, instead there is a metal track that was cemented to the slab 16" OC and the flooring is clipped to the track with fasteners. This is where my question comes in. There is no vapor barrier whatsoever under the current floor. Do I put one down before I install the new floor? Right now the concrete under the floor is bone dry and appears as if it has never been wet (no mold, or stains, no old puddle marks in the dust sitting on the concrete) I have pulled a couple small areas up to check throughout the floor. I am concerned that placing a VB over the old floor will trap the moisture and will eventually rot the wood of my "subfloor" where the wood is in perfect shape right now with nothing there. Should I leave well enough alone and not put down a VB? Any ideas?

YES! Doesn't matter if the slab looks dry, or has been dry, it doesn't mean that it always will be...install a VB. But install it over the slab, not over the existing wood. If you install it over the existing wood, and moisture is present, you are just going to trap that moisture in and deteriorate the wood beneath the barrier.

While in general, I agree with BW's analysis of the situation, you must also consider that the slab has in excess of 55 years of dry history.
Since you are however, going to remove everything down to the slab, the added cost of putting down a vapour barrier is minimal compared to the overall cost of your project. So why not?

The clip system you describe is unusual given the age of your house. Is the floor system what was originally installed at time of construction? If not, could there be an earlier floor that was removed due to moisture? You do need to consider all of these things before omitting the VB.

Thank you for the quick replies! I think I may have omitted something in my initial post. I will be installing the new hardwood over the existing hardwood/lauan that is already down. The way the slab was poured has left the slab in between the tracks uneven (It looks as if the builder planned on this system so they didn't go out of their way to level out and smooth the slab) so tearing it all out an putting down wooden sleepers would be impossible.

As far as this system being newer, I can say with almost 100% confidence that this floor is original to the home due the ownership history (single woman original owner sold to another single older woman - sold to me and my wife) and the lack of any kind of remodeling (other than the carpet) in the house. This is a straight-up 1950's ranch with no "joe homeowner" repairs or remodels. That title belongs to me.

Yup then go to the second part of BW's response. He hit it on the head with that part : dont sandwich a vapour barrier between the existing wood structure and the new hardwood.

Thank you Dennis and BW! It's good to have my line of thinking confirmed by people who have much more experience with this than I ever will. Thanks again!

It's important to define "vapor barrier, moisture barrier, vapor diffuser, moisture vapor retarder ", and all the various terms we throw around.

For a system to be a vapor barrier ( moisture vapor ), it needs to be nearly impervious. That would typically call for 6 + mil polyethylene sheeting, lapped 12" on the seams and with the seams sealed. What the OP should use is a moisture vapor diffuser/retarder. This would be 15 lb. asphalt saturated felt paper or perhaps Fortifiber Aquabar B. These products allow the potential moisture vapor to slowly be released over time and not build up in the first layer of hardwood. The polyethylene plastic sheeting should NOT be used over the existing wood floor. I would suggest using the 15 lb. felt or Aquabar B.

Have you had a professional finish guy look at the original wood yet? I would before I spent all the money your going to.