All Vault hosted projects in AutoCAD P&ID use SQL Server for the master Plant database.
When a user is working in a Vault project, they are actually working in their own local copy of the project. The working copy of the project must be stored in a folder that is local to their workstation; it cannot be stored on a network drive. Further, the working project uses its own local SQLite copy of the project database.
To read more, download the whitepaper...