Allomancy is fueled by Preservation's body? How exactly does that work? And how does that interact with Atium—it's fueled by both gods' bodies?
The powers of Ruin and Preservation are Shards of Adonalsium, pieces of the power of creation itself. Allomancy, Hemalurgy, Feruchemy are manifestations of this power in mortal form, the ability to touch the powers of creation and use them. These metallic powers are how people's physical forms interpret the use of the Shard, though it's not the only possible way they could be interpreted or used. It's what the genetics and Realmatic interactions of Scadrial allow for, and has to do with the Spiritual, the Cognitive, and the Physical Realms.
Condensed 'essence' of these godly powers can act as super-fuel for Allomancy, Feruchemy, or really any of the powers. The form of that super fuel is important. In liquid form it's most potent, in gas form it's able to fuel Allomancy as if working as a metal. In physical form it is rigid and does one specific thing. In the case of atium, it allows sight into the future. In the case of concentrated Preservation, it gives one a permanent connection to the mists and the powers of creation. (I.e., it makes them an Allomancer.)
So when a person is burning metals, they aren't using Preservation's body as a fuel so to speak—though they are tapping into the powers of creation just slightly. When Vin burns the mists, however, she'd doing just that—using the essence of Preservation, the Shard of Adonalsium itself—to fuel Allomancy. Doing this, however, rips 'troughs' through her body. It's like forcing far too much pressure through a very small, fragile hose. That much power eventually vaporizes the corporeal host, which is acting as the block and forcing the power into a single type of conduit (Allomancy) and frees it to be more expansive.