Where are all 40 vast white quills?

Well,you have to talk to mandibuzz. If you have not befriended her you will have to give her 30 everflames. Then she will take you to a shrine and behind the shrine is the quill.

There is a vast white quill in cove town, beat the Corphish. Use Oshawott to swim behind the boxes.
Go in the open vent forward until no more. Then head left, strait, left... TA the!

I all ready got 32 and I got the game 4 days ago.

In the tech area train depot on the trains.

There's 40? I only have 27 but I remember 1 being somewhere in Wish Park... And you only need 20 for Reshiram

There should be 5 in Seasong Beach, 3 in Cove Town, 3 in Windmill Way, 5 in Verdant Court, 4 in Rockridge Fort, 3 in Colosseum, 3 in Train Depot, 3 in Unawarehouse, 1 in Scientorium, 3 in Lakeshore, 3 in Wish Park and 1 in Wish Palace. There is also one on a path called "Firstfoot Path". To get to Firstfoot Path, you go to Seasong Beach. Then go to the path leading to Cove Town. Before going to Cove Town, there's a sign right in front of the exit. Read it and it asks you if you want to go to Firstfoot Path. Say yes. You then go all the way to the end of the path. There should be a Vast White Quill there. I hope this helps.

After having the game for years (ever since 2012) I can try and answer some to the best of my abilities.
It is recommended to finish the game to find them all.
I will try and constantly update this resource but I recommend looking around for them yourself, especially in the chest areas such as Seasong Beach Lighthouse as a example.

- 1) Wish Palace, close to the Zekrom "mural."

-2) Cove Town Sewers (Near Corpish, you get there by using the water to swim with Oshawott to get around the crates and head into the pipe line. Down there you will find one.)

-3) Seasong Beach Grassy Ledges (The ledges you go up to eventually find Tornadus's Shrine, you will need Snivy to jump high enough.

-4) Seasong Beach Cavern Tree Area (The place where Watchog guards a chest for you to get past him, there is one above the chest.)

-5) I believe there is one in Seasong Lighthouse, you will need all 4 pals (Snivy, Tepig, Oshawott and Pikachu to reach it I think.)

-6) Covetown Behind Building (There a few crates that with Snivy, you can jump up on and across the roof to reach a hidden area where you can later encounter Kecleon if you finish the game. There is one around there.)

-7) Cove Town Building Roof (There are crates in the alleyways where Trubbish often is, and where Espeon does her race course where you can climb up with any Pokemon. On the roof, there is one and Munsharna can also be found there along with a Rotor Motor for Misdreavus's camera in the box.)