Wasn't able to post to the existing Vertical Response post, so opened a new one. This isn't a plugin add-on, so not super elegant and requires a little more sleuthing in your own set-up, but is working for us using cURL.
I used the cURL part of this solution about a year ago, inspired by this information:
To integrate that into a Gravity Form, wrote a function that can be added to the theme functions.php file and used by the GF post submission hook:
Things you need to know to make this work:
* Your Vertical Response form id
* Gravity Form field ids (findable in the blue title bar of each field when editing your form)
Add this function to your theme's functions.php file:
* You don't have to send all of the gf fields to VR, just put the ones you want in the $postFields
* The gravity form (gf) fields that you're wanting to post to VR need to match up to fields in a VR opt-in form
* Just because you have xyz field in your VR general field list doesn't mean it will take it from your gf. The VR opt-in form has to have the fields you want to post to in it or it will cause an invalid form error from VR
* If it's successfully posting with cURL but not showing up in VR (check the $response form output and post - might be an invalid form error)- you may need to make a new opt-in form instead of using an existing one
* The gf field ids need to be in quotes when defined if you reference something like a checkbox: $gf_field_id_optin = '11.1';
If you work out an improvment to this or have a better approach, please share!