What you are showing is a MAIN Spring vice. It is not used to install a SEAR spring.
To install the sear spring, put the screw in the spring hole and start the screw in the lock plate with the spring standing up right. Thread the screw in a couple of threads and then rotate the sear spring into position on the sear, making sure that it settles onto the sear correctly. Then tighten down the screw fully. It's all done with your hands, no vice is required.