Yes, there is one entire soul trapped in that tiny glass vial. They're really quite compressible, the big problem was finding a medium that would hold the little buggers. Sure, a contract signed in blood works. But, where's the art in that? Not to mention that they tend to degrade a bit over time on paper.
Acquiring the souls was actually the easiest part of this whole ordeal; people are just giving them away. Now, finding a good way to keep hold of them, that was tough. Eventually, I had to ask for help from a certain individual who wishes to remain unnamed. Unfortunately, contract obligations prevent me from sharing the exact procedures I used. This is likely a good thing, as some of the steps involved can be quite dangerous.
For those brave souls(pun intended)who still wish to learn how it's done; I may be willing to work out a trade. (Insert more evil laughter)