Learn English – What exactly does “All Items Not On Sale” mean

'Wherein' is generally only used in formal writing. The Oxford English Dictionary gives the following senses and quotes. I have given the most recent quote, from which you can tell that the word is no longer generally used.

1. In what (thing, matter, respect, etc.)?

   1891 - Oh, Britannicus! wherein have we offended?

2. In which (place, material thing, writing, etc.)?

   1888 - Peering keenly into the shadow wherein she stood

3. In, at, during, or in the course of which (time).

   1819 - Length of days Wherein to act the deeds which are the stewards Of their revenue.

4. In which (matter, fact, action, condition, etc.); in respect of which.

   1889 - There began a round of pleasure for Julia wherein she was the central figure.

5. Into which.

   1585 - We came to an anker very nigh the castle, wherin when our patrone would followe vs [etc.].

1. "All is not lost" does not suggest that nothing is lost. It is said to counter the statement or belief that everything is lost, despite appearances to the contrary. It expresses exactly the meaning you say you want to convey, i.e., "at least one thing is not lost."

2. "Not all is lost" is simply another, more emphatic way of saying the other. It emphasizes the negative and is a stronger counter to any contrary assertions that have been made.

That's the meat of the matter; now to the seasoning: No. 1 can also seem more optimistic, and may be used to suggest that victory is in fact possible. Similarly, No. 2 may sometimes be used in a bitter, pessimistic way to point up how little is left. It may mean that everything of importance has been lost, but there may still exist a consolation prize, meager though it may be. "Our house burned to the ground, but at least the dog house remained untouched," one might say ruefully. "You see? Not all is lost."