I would prefer to buy single sessions instead of buying the whole vault, because I am only interested in 3 to 6 topics, not all 24.
@iskelzor Thanks very much for your question, and my apologies for the delayed response.
I completely understand why you’d feel this way, but unfortunately we don’t have such a policy in place. A big reason why is that the entire conferences resources are made available for free to all of our users whenever the next conference is held. You can get access to all of last years conference materials for free at this link 
I hope this helps!
All the best!