I guess that 'medium' refers to the material on which a work of art has been made. 'A miracle is being worked' plays on the dual meaning of 'work' in this context: the working out of a miracle, and the creation of a work of art. Paper, card, wood, fabric, etc are examples of media in the production of artwork. Each of these is a 'medium' [a countable noun]. The particular medium referred to is one that may quickly dissipate, making it unlikely that the artwork will endure - that would be a miracle. All the above is guesswork on my part, and the reference may be quite different, but the logic of the expression would still apply.

"so evanescent a medium" = "such an evanescent medium" "so delicious a dinner" = "such a delicious dinner" "so difficult a language" = "such a difficult language" ... and so forth. The difference between using "so" and "such" is that "so" is an adverb while "such" is a pronoun. Since "so" is an adverb, it is able to directly modify the adjective ("evanescent") whereas "such" can only do that indirectly.

The standard construction is: It seems unlikely that a miracle is being worked on a medium which is so evanescent. There are multiple media. ‘A medium’ tells you it is one of those. ‘So’ tells you that it has a high degree of evanescence.

As you may have noticed, 'medium' has several different definitions. It would be easier to answer this question if you provided more context.

In that sentence, the "a" before "medium" is used to indicate that "medium" is being described in a particular way. The phrase "so evanescent a medium" is a stylistic choice, often seen in more formal or literary English. It emphasizes the quality of the medium by placing the adjective ("evanescent") before the article ("a"). This construction is a bit unusual in modern usage, but it adds a rhythmic and poetic feel to the sentence. Essentially, it could be rephrased as "a medium that is so evanescent."