Again, my apologies, there are several threads with this topic, and I thought this had already been covered.
You do *not* want to do this in the httpd.include file. Plesk overwrites this file.
If the vhost.conf does not exist, then you will have to create a new file.
If the vhost.conf file does not exist, then make sure you have already created a subdomain in the Plesk CP first, and verify if the file then exists.
Otherwise put the following text into a new file and save it as vhost.conf
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteMap lowercase int:tolower
ServerAlias
www.sub.domain.com
RewriteCond %{HTTP_HOST} ^
www.sub.domain.com$
RewriteCond %{REQUEST_URI} !^/subdomains/sub/
RewriteCond %{REQUEST_URI} !error_docs
RewriteRule (.*) /subdomains/sub/$1
</IfModule>
Of course, change all references to the actual sub and domain you are creating.
Too many days and nights without sleep, too many posts in too many forums to keep track of....