Minimum would be 3/8". You already have the strength in the planks. You only need the plywood because it's required that you go ov er engineered wood panels with any kind of tile underlayment, whether it be cement board membrane or other. Now, two things-- first, cement board will not hide rolls in the floor (I'm going through the same thing with my house-- it's 101 years old). It'll only conform to the shape of the subfloor. Your best bet would be to use jacks (adjustable lollicolumns) to straighten out the joists (I'm assuming here you don't have a finished basement! :-) ) The nice thing about using the jacks to straighten out the joisting is it would also give you the added support you'd need for a stone floor at the same time. :-) If you don't want to go that route, there's always self leveling compound OVER the cement board or membrane.

I do not believe AC plywood is suitable for anything vinyl as it is not a solid core plywood. It used to be vinyl had to be over luan plywood although I am sure that has changed today but I still do not think AC plywood is acceptable.

I agree, it is odd. Is the maple flooring actually flooring and not a subfloor? I would be tempted to pull up some of the maple flooring to see what was under it and get a better idea. However, if it is good flooring and you want to use it, then you will need to use some sort of transition between that and the higher areas. Here in Colorado floors are sometimes made with in-floor heating which is covered with a thin layer of cement. That seems unlikely in CA, but maybe that is what you have. --Kris

actually 3/4 inch plywood is recommended but particleboard is ok if rated a PS2-92. 3/4 or 22/32 is recommended for particleboard as well. I'm not an installer but I believe the subfloor if level would be ok as long as you weren't gluing or stapling. If your floating the engineered floor the subfloor wouldn't be as big an issue, but glue or nail requires a better subfloor.