How would a human body decompose in vacuum?
I doubt that it would decompose.
There is no oxygen in space, so aerobic bacteria that normally decompose bodies will either become dormant or die.
The temperature is colder than you can ever imagine, -157 C (-250 F) on the shady side in near Earth orbit, and +121 C (+250 F) on the sunny side (measured at the ISS).
So in the shade it would be too cold for bacteria to degrade the body while in the sun, you still have the lack of oxygen problem.
So what would happen would be that the body would desiccate, or dry up like a mummy. That's because as it freezes, water expands and would break down the tissue, but as the body rotates into the sun again, the ice that has made it to the surface would sublime off into space. The body would therefore slowly dry up with the freeze/thaw cycles and perhaps the surface at least would become crumbly from the action of the water. This process would continue until either the body ends up in a place where there's no temperature difference, no rotation, or all the water is gone at which point nothing more would happen.
But that's not really what is usually meant by decomposition.