Vimeo API - generate unauthenticated token with WP HTTP API - php

Question

I'm trying to get an unauthenticated token from Vimeo's current API v3, in order to use it to get a simple list of my own videos on my own website. I'm using a WordPress HTTP API function 'wp_remote_post' to generate a proper http request.
According to Vimeo, this is the correct way to do this and it's done with a POST request. Here are the arguments:
HTTP Method: POST
HTTP URL: api.vimeo.com/oauth/authorize/client
HTTP Headers: Authorization: basic, base64_encode( "$client_id: $client_secret" )
Request Body: grant_type=client_credentials&scope=public%20private
and getting
[body] => {"error":"You must provide a valid authenticated access token."}
[raw] => HTTP/1.1 401 Authorization Required
Why is Vimeo asking for a valid authenticated access token on an explicitly UNauthenticated call? I have provided the actual client id and client secret from my Vimeo app, using Vimeo's instructions to receive an unauthenticated access token. I'm sending the request from my local environment.
I have checked the similar question How to generate Vimeo unauthenticated access token? and have followed everything outlined there. No dice, and i've been trying to do this for hours.

Answer 1

Vimeo API seems to only accept the parameters as part of the query string, not as part of the HTTP POST object ('body', 'data', or other).
Only when I coded the parameters directly into the URL, rather than passing them as parameters in the post object, the post worked.
Works:
$url = 'https://api.vimeo.com/oauth/authorize/client?grant_type=client_credentials&scope=public%20private';
$auth = base64_encode( $developer_key . ':' . $secret_key );
$headers = array(
'Authorization' => 'Basic ' . $auth,
'Content-Type' => 'application/json'
);
$args = array(
'headers' => $headers
);
$response = wp_remote_post( $url, $args );
Does Not Work:
$url = 'https://api.vimeo.com/oauth/authorize/client';
$auth = base64_encode( $developer_key . ':' . $secret_key );
$data = array(
'grant_type' => 'client_credentials',
'scope' => 'public private'
);
$headers = array(
'Authorization' => 'Basic ' . $auth,
'Content-Type' => 'application/json'
);
$args = array(
'headers' => $headers,
'data' => $data
);
Hmmm. As of WordPress 4.6, the WP_HTTP class is now built on Requests by Ryan McCue.
So it seems my question is really also a question about how wp_remote_post() constructs the request. There does not seem to be a way to pass the parameters to the function and have them stringified in the URL.