I don't have the books anymore (library loan) but why is it that all the vampires had forgotten that Spirit was an element? Or was it that no one ever knew it was an element?

I think no one really ever knew about it.
Like Vladimir. I don't think anyone ever really put it together when he was alive. And before Lissa and Adrian all the other spirit users either didn't know what spirit was or kept it hidden... I think.

I have a vague recollection that once the Moroi decided to turn pacifist, they lost touch with some of their abilities and knowledge base.

spirt is reallt rare so not many people know about it. maybe people knew about it before but now they completely forgot about it

Although, now that Adrian and Lissa have put their powers out in the open (after the attack) do you think there will be an increase in the number of spirit users? B/c didn't alot of them just think their powers never really developed?

Exactly. They should check the Moroi Sanitarium. Half the residents may be Spirit users.

I think that it was to powerful so they forgot about it on perpose because you can see into there heads while they dream heal and alot of other stuff

Well my guess is that it was always the least popular element. Say only 1 out of 10 moroi used spirit maybe. So I guess it is possible that it may have gradualy become even less specialised to the point that it was forgotten.

That is about as good as my guess gets.

Lol. I love the comment about the Sanitarium. And I couldn't agree more. They should now add it. Probably half the people that 'didnt have powers' were spirit.. Okay, maybe not half but some of them. They might not be as great as Lissa or Adrian, but who cares? They DID specialize. I agree with the statement, also, that since it was so rare that might be a factor of why over time it faded out.

I think the reason it was forgotten was because it was so rare. I know they now have something to look for, a sign that someone has Spirit (not specialising), but it seems as though that can't be the only sign. Then again, maybe not.
I like the idea of checking the Sanitarium, they might indeed find some Spirit users in there, but we can't assume every crazy person has Spirit. Some spirit users are likely to be living full lives, Adrian proves that; he found a way to battle depression and the craziness without having a shadow-kissed friend; so who's to say there aren't others out there?

My guess is that it's extremely hard to find. Not all spirit users develop the same powers at the same stages, so while one can heal or walk through dreams etc., it'd be hard to identify. And the extra Compulsion abilities could just be seen as normal, especially if these people didn't have another power. They have nothing else, so all their energy can go into compulsion.
I also wonder whether Spirit has different levels of strength. Are Adrian, Lissa, Ms. Karp and St. Vladamir the exceptions, or are all Spirit users that strong?
And was Spirit ever all that well known to be forgotten in the first place? You'd think that if there was a 5th element then it would have been written down enough to follow throughout history. The other elements were continued and well researched. It had to have been a really rare element to be forgotten.

We need more info on what else Spirit-Wielders are capable of, since they all start with a different Spirit Ability, apart from detecting Auras, at least!

My theory about the way spirit works is simple. Spirit has different abilities and it manifests differently in each person. They are all capable of doing the same things and can achieve it to some extent but are extremely powerful or gifted in one area or aspect.

For instance, ( without spoilers)

Adrian has his strong suits while Lissa has her.