Note that SELinux and traditional Unix file permissions (and optionally POSIX ACLs) are two separate systems. For SELinux, it will be easiest to think it as a per-service access limiter. It can be much more, but as far as I know, this will be the easiest to set up with the RedHat standard SELinux policy. If the CMS is PHP-based (e.g. Joomla), then its write operations are started by the child processes of httpd, so the SELinux type you need for it will probably be httpd_sys_rw_content_t.

In RedHat, all the system services have a _selinux man page, which describes the relevant SELinux types and booleans. In your case, you'll want to read "man httpd_selinux" and "man ftpd_selinux". These man pages are in the selinux-policy-devel RPM package.

To allow httpd and its child processes (and no other system service!) write access to /var/www/vhosts/web1:

# semanage fcontext -a -t httpd_sys_rw_content_t '/var/www/vhosts/web1(/.*)?'
# restorecon -R /var/www/vhosts/web1

If you want the files to be writable by both HTTP and FTP services, then you'll need to use the public_content_rw_t SELinux type instead of httpd_sys_rw_content_t, and enable two SELinux booleans:

# setsebool -P httpd_anon_write=1
# setsebool -P ftpd_anon_write=1

Now SELinux will allow writes, but the traditional Unix file permissions still need to be addressed. To allow access to /var/www/vhosts/web1 by both the CMS and the webmaster user, you'll want to identify the user the CMS processes are run as. For a PHP-based CMS, it will probably be user that runs the httpd service, i.e. "apache".

Create a group, and add both the webmaster and the httpd service users to that group:

# groupadd vh_web1
# usermod -a -G vh_web1 apache
# usermod -a -G vh_web1 <webmaster_username>

Set the group ownership of the files in the directory tree to that group:

# chgrp -R vh_web1 /var/www/vhosts/web1

Grant group write permissions for all the files and sub-directories in it:

# chmod -R g+rw /var/www/vhosts/web1

Grant setgid permission to all the directories and sub-directories in the tree, so that the group vh_web1 membership will be propagated automatically to all new files and directories in the tree:

# find /var/www/vhosts/web1 -type d -exec chmod g+s {} \+

That's it. If you are using the RedHat default umask value of 0002 for regular users, the webmaster user should not need to do anything special: any files or directories s/he copies or creates under /var/www/vhosts/web1 should automatically get the right permissions and SELinux labels.