A boom vang isn't necessary. I haven't used one for 4 years or more. Don't bother.

Getting the hook to work right just takes a lot of patience and experimentation. There are many different ways to tie the ring on the end of the halyard and you just have to keep trying until you find the right one. Try pulling the sail up, tip the boat over and then experiment. Better still get someone else to pull on the halyard while you watch what happens at the top of the mast.

If the sail has been changed make sure that it can go high enough for the ring to get on and off. Sometimes the cut-out at the head doesn't suit the pulley/hook arrangement.