You can play the hand parts very slowly but inaccurately, the pedals remain a mystery to you and putting it all together - it simply goes over your head. You love listening to this piece but seem to have stuck in choosing the right articulation - which notes should be detached and which - played legato.
What to do? Can you still learn this fabulous piece, can I give you some magical words of advice which would help solve all your technical and mental challenges you would meet in the Final?
Yes, but probably not the way you expect.
You see, if this is the first piece of Vierne you have ever practiced which is quite likely, then I would recommend mastering first some of the easiest and slower compositions of the same composer.
But you might say, that you love only the Final so much that it's not worth the time and effort to learn the easier pieces. In other words, you dream of playing the Final and nothing else.
But I think it's worth it, if you're serious. If you're learning the organ playing just to show off, then of course you want some flashy sounding fast and loud toccatas to impress others.
However, there's no one to impress, really but yourself because nobody cares.
Therefore, I recommend you take up 8-10 easier pieces before attempting to play this Final, from Messe Basse, Op. 30, Allegretto, 24 Pieces en Style Libre, Op. 31, and ALL other movements from the Symphony No. 1. All of these compositions can be found here.
Practice and master these pieces. And perform them in public to know just what it takes to get ready for this Final.
[HT to Andrew}