# search for roundcube config file
find / -name *roundcubemail.conf*
# search for installed packages
yum list installed |grep roundcube

# is enough to disable roundcube
rm -rf /etc/httpd/conf.d/roundcubemail.conf

# seems to be no problem as well to completely uninstall it
# vestacp works just as well without it
yum remove roundcubemail.noarch


VestaCP does not provide an own roundcube version, if you want to disable roundcube, you can to this for example by removing the /webmail alias from the system:

rm /etc/apache2/conf.d/roundcube.conf
service apache2 restart

if you want to re-enable it, you can create again the symlink:

ln -s /etc/roundcube/apache.conf /etc/apache2/conf.d/roundcube.conf
service apache2 restart

This will work for Ubuntu and Debian, should be a similar way for CentOS/Redhat systems. (src)