How does the photographer know what the couple want?
The photographer should go through a list of shots and discuss it with them. Then when the couple say we really want a shot of the rings, vows, kiss, whatever the photographer should be saying ok, have you asked the vicar about this? If not I suggest you do as some can have very strict rules about photography during the service. Make it clear these shots are important to you. It will sound a lot better coming from you and carry a lot more weight than if I just ask on the day. If we plan this properly then we can make it happen. I expect the vicar will have some questions or concerns and I'm happy to phone or meet with them prior to the wedding to work out a plan of action and put their mind at rest.
If the vicar still says no at least the couple are aware of it long before the wedding day and we have a chance to think of alternative ways of getting the shots. Stage them after the service, for example.
Just turning up and winging it will lead to disappointment.