Asher Mir’s arm was not turned into a Vex arm, it was cut off and replaced with a Vex arm surgically. We don’t know if that arm is carrying the infectious Vex cells in it or if it itself is made of the infectious cells, so we don’t know if the Vex are actually capable of taking over a host body Kabr did not, to our knowledge, physically join te Vex in any way. He suffered from some Vex physical and mental degradation and then drank some Mind Fluid, which drove him insane but allowed him to have a general sense of what the Vex were thinking since he literally had their brain inside of him, and then he made the Aegis and died. A fragment of his conscience may have been snagged by the Vex Collective Mind, but Kabr never became Vex