This piece is a kind of a compensation for the fact Chopin didn't compose for organ ; Vierne liked Chopin much and played it often at the piano.
The dynamics don't occur exactly where Vierne specified them, but at the nearest "comfortable" place considering the notes to play at the pedal.
The score doesn't mention to slow a bit the tempo for the two intermediate chordal parts, but I guessed the the indication "molto cantabile" could legitimate it ?