I was wondering if i Buy bo6 vault edition on my bothers xbox but on my account will i...

It doesn't matter where you buy it as long as you're signed into your account. The rewards will be tied to your account so you will get them.

And I'll buy the vault for bo6

No i bought the vault edition on my xbox one and the account from my xbox one is linked with my activision and steam account and i got warzone 2.0 from steam and when i played warzone on steam i couldnt use my vault edition operators

my brother buy the cross gen edition and we share since years,i buy the vault edition upgrade and i get nothing no skins and no operator,i cant see the operators from the vault edition anymore,is it not working? i guess i will my money back but in my order history it insnt aviable yet

This issue is likely related to how Xbox handles game licenses and ownership. When a game is purchased, the license is tied to the Xbox account that made the purchase. Sharing games between multiple Xbox accounts is possible through the "Home Xbox" feature, which allows multiple users to share access to a library of digital games, but it has some limitations.

It sounds like in your case, the Vault Edition of Modern Warfare 2 is not showing up as available for purchase on your account because it is not the account that owns the license for the base game. To resolve this issue, you may need to purchase the Vault Edition directly on the account that owns the license for the base game. Alternatively, you could consider setting your brother's account as your "Home Xbox," which would allow you to share his game library, including any DLC, across both consoles.