Why do we consider only the centrifugal force while measuring tension in string in vertical rotator motion “” but not the real centripetalforce ?

Remember centrifugal force is a pseudo force. And pseudo force is applied in non inertial frame. In vertical circular motion body is under acceleration. Therefore if u make your observations from body u apply a pseudo force to make correct equation. But if u make your observation from ground u have to consider the centripetal force and make equations.