I don't think the Larian Vault version will be able to directly link to Steam. The other variation I've seen recommended to avoid re-installing the Steam version is to start the install and then pause it, move/copy the non-Steam install folder to the Steam location, then in Steam verify integrity of the local files.
In general it is better to buy directly from a developer's store. AFAIK Steam doesn't charge developers anything to create keys.
EDIT: I'm getting larian vault as distributor. Thanks Raze!