I am new to vRA and wanted to know if it would be possible to integrate with third party vendors like fortigate,paloalto and cisco firewalls inorder for IaaS admins to create and publish policies. Also, it is possible to integrate with network devices like Cisco ACI, Juniper QFX for network provisioing
That's going to depend on what you're trying to do. There are lots and lots of ways to "integrate" with something and so the sample code will reflect that. Again, if it has an API you can integrate with it. Check the Hands on Labs for some demos you can use that show this concept.