Probably, though it depends on the specific deal they've signed with Steam. I'd still bet that Larian gets the most money out of a Larian Vault purchase.
Original Sin might be different in the way they've worked out the distribution.
You don't have to bet on it because it is a sure ball that Larian will get most money if we buy directly from the vault. In the simplest terms, they just cut off the middle man or the retailer in the view. Hence, able to maximize the profits especially if the consumer buys a digital product without a headache of logistics being a burden to the developer.
If you want to support the developer fully, its better to buy directly from them than use third party source. There's a blog swen made about it and it really gives you a general idea on how much they saved.
