I made migration from vBulletin 4.x but before that i forgot to set a decent thumbnail size in my xenforo. Results is micro thumb.
Now i set a decent new size for the thumb and i wish to rebuild the imported thumb.
Could repeat only the attacchement import step be a solution ?
I don't have a solution for you. There is no thumbnail rebuild utility, and you can't run the attachment module again without doing the entire import again. If this is not reasonable then you can post a request for a thumbnail rebuild addon: