Today I also tested my sft for WF Mille Feuilles. Mille Feuilles is a fancy name for a vanilla slice.

Basically it's a crisp puff pastry with a vanilla custard-like filling with an icing on top. There are many different recipes depending on where in the world it's made.
They look like this...
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...and also like this.
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I tested this at 3% diluted in 70VG/30PG 6mg nic using a 26/36 SS316L clapton with a Hussar v2 RDA (MTL/RDL) on a Mono SQ DNA 75 at 16w-30w. 4 week steep.
I've never had a Mille Feuilles in real life so I don't know how accurate this flavor is compared to a real vanilla slice.
The vanilla patisserie cream is light but very smooth. It's almost like the filling inside of an eclair (in part) but not quite that in full measure. The vanilla note is light. It is rich tasting but not in a very heavy way like some creams are. It has a light delicate pastry note much like a puff pastry. There is a sweet bakers icing note much like the icing shown in the pics above might taste.
How sweet is it? It's not something I would recommend to someone who does not like sweet flavors unless they know how to tame the sweetness. Personally I don't care for real sweet vapes or flavors. But, if I am able to tame the sweetness of a flavor then I am fine with them. Someone who likes sweet flavors would probably enjoy this in a mix as is and may even enjoy it as a single flavor.
I do not taste anything off putting. It is a good smooth flavor with no harshness. I see it's use as more of an additive to a recipe. I will use it from 0.5%-2% (depending). Some may find it to be nice as a main flavor (2%-3%+).
The pastry note needs help in order for it to shine as a true pastry. My croissant base would work well for that.
I can see it being used as a patisserie cream but do feel it needs the help of another cream or a custard. Pairing it with FA Custard Premium would be excellent. Together, with perhaps a light touch of INW Shisha Vanilla, they would make a convincing eclair filling. I would suggest CAP Vanilla Custard v1 if more eggy notes are desired. Custards should be used with a light touch so as not to overtake Mille Feuilles.
Mille Feuilles would also work nicely paired with fruits, bakery, chocolate and vanilla. But keep in mind this is a delicate flavor.
As always, YMMV.