How do I analyze the vault with the structure?
After defining the vault in the required position and geometry in your project, activate the Set Vault Restraints command in the Vault auxiliary toolbar.
The Set Boundary Conditions dialog will open.
In the dialog that opens, after marking the boundary conditions for solution with building system, make your arrangement so that only the option/options that are supported on each side of the vault will remain active.
If you want to solve the vault element with the structure, it would be a better solution to select the Use semi rigid diaphragm option from the Analysis Settings dialog, General tab.