this is not a completed timbrel vault but a quick experiment. After a first try the second went better and then i started to get into it.
I used gypsum wall plaster. Have no idea if it is as good at pure gypsum, but once you complete there is no strength in the plaster anyway but opposing forces.
I would butter two ends, put it in place for like 5 seconds, then lightly lean a stick against it. then move to the next brick. building one more on each line at a time.
If you bang on it is like a drum, and i really bang hard on it.
I need to make one vault like this, then i put mortar between the next course and the third. I is supposed to be stronger than just a brick as thick as 3 thin ones. Like plywood.