The reason you are getting an error is that |
symbol is not allowed in the proper URI, hence it needs to be escaped before it's parsed. URI comes with a method to do it for you:
uri = URI(URI.escape "https://graph.facebook.com/#{VANDALS_ID}/posts/?#{fb_access_token}")
uri.to_s #=> https://graph.facebook.com/id-here/posts/?access_token=123456789%7Cabcdefghijk
When the url is requested, the server should automatically decode it, so all should work as expected.