The 'Buy on Steam' are just Steam keys, while the Humble widget provides a DRM free download (from the Humble Store
) in addition to a Steam key. It is also available from GOG.com, DRM free.
The store function of the Larian Vault was shut down early last year due to changes in the EU tax laws concerning digital goods, so there are only widgets there to purchase through other distributors.
If you are looking for the retail release (which is Steam), your best bet would probably be online (Amazon, eBay, etc).