Well, every Kickstarter backer that had the option of getting a non-Steam DVD has both the Steam and GOG versions of the game. It would be much easier to just download the full GOG installer than try to get the patches to install over the non-Steam DVD, though (this will be even more the case after the next large update).
Getting the list of differences to make a patch isn't hard, though it is an extra step to create the patch. The time consuming part is testing each patch against all previous install configurations, with a variety of new and previous saves.
There is no Larian Vault version of the game. In fact, due to legal changes in Europe awhile ago concerning digital goods (I assume tax related; didn't ask), the Larian Vault no longer sells games directly (people can still re-download previous purchases). There are just Steam and Humble widgets to buy games (and in the case of D:OS, the Humble Store version is just Steam, without a DRM free download).
After the game is finished being updated, some kind of non-Steam retail re-release is theoretically possible.
