As the title states. Stephenson uses (and describes the use of) the word "vitamin" to mean any goods that can be shipped to space. Is that actual space lingo?
I believe Stephenson may have picked that up from the RepRap community in the open source 3D printing world. "RepRap is about making self-replicating machines."
At the present state, of course, there are many components that can't be reproduced (yet). The RepRap community calls these Vitamins. "In RepRap jargon, a vitamin (also called an imported part or non-printed part) is anything that you need to make a RepRap, which a RepRap cannot itself make."
This is very similar to the usage in SevenEves. They're shipping up components that can't be (or can't easily be) produced in orbit.