I was compiling this for someone else and thought I would share:
Original Trilogy were single books. Then Dark Reunion was added.
When they were reprinted the first two books were reprinted together then book 3 & 4 were reprinted together. I have only found these in paperback and not hardback.
When the tv show came out all four were reprinted as separate books with the tv characters on the front covers. In my looking it looks like the Return trilogy has also been printed in paperback form with the tv characters on the covers.
I have also come across two new versions of the first four in reprints plus paperback versions of NightFall and Shadow Souls. These versions appear to have come out in 2010.
[link]There are also paperback version of Nightfall and Return that have the same covers as the hard back versions.
Here are the first four vampires diaries books in German.
[link]