I have used below settings in my java code for setting up the Proxy host and port to access this Vision API using ImageAnnotatorClient.
System.setProperty("https.proxyHost", "HOST IP ADDRESS");
System.setProperty("https.proxyPort", "HOST PORT NUMBER");
Google API allows only HTTPS proxy settings. I tried with setting up http.proxyHost hasn't worked out. Try these settings. It worked for me.
Thanks