This is the beautiful thing about VirtualHostX. Since it's a virtual machine, you're free to login and make whatever changes you want. This includes turning on mod_rewrite...
First, ssh into the virtual machine by clicking the "Terminal" icon in VHX's toolbar.
Then run these commands....
sudo a2enmod rewrite sudo service apache2 restart
You may then need to enable the feature on each of your website's individually by adding the appropriate Apache directives (commands) to VirtualHostX's Custom Directives box.
First, select your website in the VirtualHostX sidebar and then add the following commands to your Custom Directives box...
Options FollowSymLinks AllowOverride All AuthConfig Order deny,allow Deny from all
After adding those settings, click the "checkmark" toolbar button to save your new settings and restart your web server.
Once you've done that, you should be able to add your mod_rewrite rules as you normally would in your website's