"if i have cape mod my run will accept ?"

No.

"how can i get custom cape in vanilla"

You can't.

if you migrate your account then you can

or just turn it off

^ If you have mc currently, which should be everyone here, you can get custom capes. Look up on YouTube "custom capes from migrator cape" or smth like that. You just have to mess around with a few files.

Signed up with a Microsoft account moment

im trying to delay it until they force me to

u can do it if u migrate ur account and edit some files and launch mc with multimc