I've found every other tome in the game except for this one. Supposedly (as in: I've read it in the official guide) a second merchant in Haven sells it (the first one certainly doesn't). I tried both outcomes of the ashes quest and went back to Haven to search for it. In both cases, some of the villagers had returned but no merchant. I did it at a relatively high level, I tried returning after the landsmeet, nothing. The only thing I have not tried is to not kill the first shopkeeper and then do the ashes quest.
I just want to know whether anyone actually found this tome. If you did, when and how?
PS: Overall I found the official guide quite bad for min-maxing characters. It lists incorrect information even for the tomes I found, and it forgets to tell you for example that you can get two cheap backpacks in Ostagar. It would not surprise me if the Greater Tome of the Mortal Vessel does not even exist in the game, and that the rumor simply spread from people reading the guide.