Thought I'd just let you know about something I discovered. I am able to display Vungle ads directly with the Vungle extension, so my credentials seem fine. But Heyzap was not displaying anything from Vungle. I just found this on Vungle's website though (related to Test/Active Modes):
Test Mode - This is the default mode. Use Test Mode if your app is not yet live in the app store and you want to send test ads to devices. Vungle can send an unlimited number of test ads. You do not earn revenue while your app is in Test Mode. You also cannot link it to any third-party services, such as mediation partners. It is common to see ad repetition in this mode as Vungle cycles through a limited set of ads to your users.
Active - Make sure to set your application to Active if your app is live in the app store. It now shows ads to users and adheres your settings. Active mode is also required if you are using a mediation partner, such as Fyber: your app must be Active for you to link the third-party service.
So, I guess I can't really test Vungle ads via Heyzap until my app is Active on the App store. But at least I've found the source of the problem!