One of the best in the genre (SPOILERS)
I have had the DVD for a few years but only got around to seeing the film for the first time a few nights ago. On the whole, I thought that it was very good, certainly more spooky than Paranormal Activity and better than several others of the same genre. The movie probably needs to be seen a few times to pick-up all the clues but based on what I saw and the discussion here, my own interpretation is as follows:
"Billy"- the deceptively loony friend who 'introduces' John to Vera Madeline was definitely an established and active cult member and behind the careful plan to trap John. The pumpkin carving was definitely a clue, considering the pumpkin was also seen behind Rebecca in the library scenes.
I don't think that Rebecca was in it from the start. It is just possible that Billy might have 'groomed' her into his beliefs unbeknown to John but I felt that it is more likely that she was ensnared gradually soon after going to Collingswood. She was certainly fully integrated by the time she went exploring in the attic of the abandoned house. Something like that is highly unusual for even the bravest and most sceptical of young women and this should have warned John.
Vera Madeline was just what she claims she was. A loopy but genuine on-line psychic who was used by Billy as a vehicle to draw John and Rebecca into the trap. Vera only realised what was happening after interacting with the two and genuinely tried to help both early on. Later she realised that Rebecca was already too far in to help and this reflects in the fact that she refused to speak to her the second time. But later Vera realised that it was John who was the actual victim but by then it was too late.
John's insecure and gullible personality is perhaps what led him to be picked as the next sacrificial victim. That and Billy's familiarity with John's house and habits point to that. Although it was Rebecca who called Vera first, it was only after John's request and so we can assume that he was spooked by it right from the start.