It seems to not work.
For example the FTP service cannot be separated easily:
* ISPConfig creates the users (like web1) and groups (like client1) in the Web server's VE (www) as normal system users (/etc/passwd and /etc/group), so they can't be used by pure-ftpd in the ftp VE easily. There are tricks, certainly, but we would get an unsupported configuration then. (ISPConfig does not use PAM for the actual system users.)
* ISPConfig stores the server_id of the Web server into the ftp_user record, so it cannot login through FTP without hacking the pure-ftpd configuration files or ISPConfig.
I've failed to separate the services while preserving ISPConfig's central management.