How "jailed" is jailed enough? Running each "site" in an own webserver with a single database server then you can probably break the jail through the database server.
Separating each site also in separate processes for database and webserver there is a small chance to break the jail through the operating system. That said leads to the question if it is the best way to have each site in an own virtual machine.
But couldn't you break the VM jail through the VM host?
So it is only a question of how secure do you want it. And how much time you want to spent making your environment secure. Where 100% security is never possible though.