Do you run your entire site in https? It seems as though that may be the only way for us to get grade book to function with this LTI.
We've already purchased VT integration but without the assignment/grading feature it is a little deficient.
Tech support from VT said, "At bare minimum, any POSTs to https://[your-moodle-domain]/mod/lti/service.php must be under SSL in order for the gradebook integration to work. We'll need to do a little testing after your Moodle host has been able to do this just to make sure nothing else needs to be done, but in theory that should work. If this isn't possible, you may not be able to use the gradebook integration portion of VoiceThread."