The VCS Plug-In uses the standard SCC API to interface with version control systems. If the version control system provides a complete SCC interface, it can be used with PL/SQL Developers VCS Plug-In.
For CVS there is a PushOK interface (http://www.pushok.com/soft_cvs.php) and an Igloo SCC interface (http://www.jalindi.com/igloo), but there may be other SCC implementations for CVS available.