OK, so you're placing a reservation for a site somewhere in a United States Forest Service
campsite, and they tell you that there is this thing called a "vault toilet".
Now to explain, there are four kinds of toilets here. There's a "pit toilet", which means that it's a seat over a hole inside an enclosure, a flush toilet (which is exactly what it says on the tin), a chemical toilet (a standard port-a-potty you would find scattered around, mostly at construction sites) and a vault toilet.
The vault toilet is a brick or otherwise semi-modern enclosure (for your privacy) surrounding a hole in the ground, which has a seat and standoff going into what amounts to a cesspool. Not a septic tank, but directly into a cesspool.
Bring plenty of Lysol.