I am driving myself crazy because there is so much conflicting information out there. I'm hoping for some clarity from folks who have been there/done that.
I'm in the Portland (Oregon) metro area, in a 15 year old home. We're replacing carpet and existing 3/4" oak on the entire main floor with 1/2" bamboo, nail-down. I can smell musty odors from the crawlspace sometimes, so I plan to replace the vapor barrier down in the crawlspace first to take care of that.
I plan to lay down 3/8" CDX plywood over the entire subfloor. Screw down with deck screws, 1/8" space between panels, with liquid nails in the seams.
Question #1: Is that plywood thickness sufficient for the flooring nails (I plan on using cleats) to get a good hold? (If I go with 1/2" it will require a reducer or other transition to tile in the laundry and bathroom, and I want to avoid that)
Question #2: What is the best underlayment/vapor retarder to use? Is 15lb felt the best choice? Is Aquabar better? should I use 30lb felt? Silicone paper? I read all sorts of conflicting info on this.
Question #3: Do I put the vapor retarder under the plywood, between the plywood and the bamboo, or both? I worry about creating a "sandwich" - if the plywood is between two vapor barriers, it seems that it could trap moisture and cause problems.
I want to do this right!! Thanks for sharing your experience and expertise on this.

I brought home some 3/8" plywood and laid it down with the bamboo - even without felt paper or some other underlayment it was still 1/8" too high, so I 'm going to need to go with 1/4" plywood. Will that work OK?
Also, as I did more prep work on the floor, I found some of the T&G that is really out of whack - a couple boards are 1/4" higher than the surrounding boards. Do I try to sand them down, or pull them up and try to fit them in so they are flat?
Can't wait to get past this part and get into the actual flooring install! But Iknow it's all about the prep work. Thanks for your help on this (please respond and give me some insight!)

Question 1
I believe that 3/8" is the typical minimum thickness of plywood for floor over floor application. You may be able to get away with 1/4" but be certain to use real plywood--not luan.
Removing and replacing the raised sections of subfloor may be your quickest fix. A hand held electric planer works well here too--just be sure to remove the nails first.

Questions 2 & 3

Aquabar under the bamboo.