In your Apache configuration file, find a line with 'DocumentRoot' and modify accordingly.
EDIT:
Regarding your comment, then you're looking for this:
DocumentRoot httpdocs/public/
Though for the virtual-hosts request,
<VirtualHost *:80>
ServerAdmin webmaster@sitename.com
ServerName www.sitename.com
DocumentRoot /var/www/sitename #STILL DOCUMENTROOT!!!
<Directory />
Options FollowSymLinks
AllowOverride None
</Directory>
<Directory /var/www/sitename/>
Options Indexes FollowSymLinks MultiViews
AllowOverride All
Order allow,deny
allow from all
</Directory>
ErrorLog /var/log/apache2/error_sitename.log
# Possible values include: debug, info, notice, warn, error, crit,
# alert, emerg.
LogLevel warn
CustomLog /var/log/apache2/access_sitename.log combined
</VirtualHost>
Though note still, it uses the DocumentRoot directive. You can have as many sites as you want. Modify the paths to match the httpdocs/public directory, and I assume that inside /public/ there will be directories for other virtual hosts?
If this isn't what you wanted, please re-phrase your question.