I installed OSQA Bitnami on my VPS. And I also point 3 domains to this VPS. Now I want each domain point to different web service (I have another PHP website I want to host here). 
How can I run php service along with OSQA Bitnami (it's python stack)?
You can install BitNami modules for each one those stacks, just go to the stack download page, select the module for your platform, execute it in the command line and point to the existing installation. Then you will need to configure httpd.conf to point each domain to each app.
How can I run Bitnami OSQA with other web service (wordpress, joomla)?