It seems there was a misunderstanding, and Vonage is allowing us to use a Password Credentials Grant Type for authorization.
From my understanding the process seems to work like this:
1. Send an HTTP Request to the /authorize endpoint.
2. Redirected to a login page.
3. Login with Vonage provided credentials.
4. Redirect to the specified redirect url.
5. The Authorization code is in the url.
6. Send an HTTP Request to /token endpoint, with Authorization Code as a parameter.
7. API responds with the access token needed for API calls.
I have been able to walk through this process in Postman, but I do not know if PI is capable of going through the redirects, and logins needed to finish the process. I have been researching this for a couple of days, and any help is greatly appreciated.