I've tried twice, and in both cases Falion didn't perform the ritual.

The first time I was waiting for him on the summoning circle. It turned out that he arrived scared, being pursued by a mud crab. I killed it and then the mission "meet Falion at dawn" was completed and "Talk to Falion" started. However there was no option to tell him to perform the ceremony. I though it could have been because of the crab, so I reloaded from the last save, and tried again: gave him the black soul gem, he told me to meet him blah blah blah. This time when I arrived at 5am, maybe a little later because he was already there, once again the mission "meet Falion at dawn" was completed and replaced by "talk to Falion", and again, he did nothing else than stand still to answer questions already answered.

I also tried attacking him in case I had to force him to perform the ritual. No result. He attacked me again and again after recovering (because he can't be killed).
I'm not at stage 4 of vampirism at this moment by the way. No one attacks at first sight.

Maybe I'm missing something??

Any help appreciated. This vampirism thing is really annoying.