It's just a simple 301 redirect. Like so:
RewriteRule http://www.somedomain.com/webmail$ http://www.actualdomain.com/webmail/ [R=301,L]
This goes on the server that has these hosted domains. Then it'll route that traffic to the actual VPS that hosts the webmail client.
By the way, well done in picking Roundcube. I'm using it on my VPS, literally is one of the best webmail clients I've come across!
Another way is to include it in your vhosts file with code similar to this:
IndexOptions +FancyIndexing NameWidth=*
Options Includes FollowSymLinks Indexes
Allow from all
I know the .htaccess method would work, I'm not entirely sure if the above vhost code would work as well though as I've never tested it.