VNC/SSH: How to execute a command after logging in?

I'm looking for a way to create a VNC console user that executes the command screen or bash immediately after logging in so they get their own restricted shell for executing commands etc. Screen/bash will be pre-configured to handle all security issues. The VNC console also has to terminate once the user types the exit/logout command as I don't want them to access the rest of the system. I have already looked through the documentation of TigerVNC and TightVNC but couldn't find anything useful. If this is not possible with VNC, a SSH console can work as well. The server's OS is CentOS.