I think you have an extra o-ring holding it out. You may have to get a replacement piece at a scrap yard. Some parts stores will install new hose and crimp it for you. I have used special hose clamps with an extra piece to prevent it from coming off. This was on R-12 systems but your best bet is a professional crimp job.
If it won't hold vacuum after removing the old o-ring get a used piece and ask someone to crimp the new piece on your hose.