Joomla templates may not allow third party html forms like the one VerticalResponse provides in your account. I have tried doing this in my own website where I take the form method tag's URL and insert that as a link in my joomla template. That way, it pops open the form you create in VR in a new window, and this should work just fine.
Be sure to go through the whole design process of the form in VR's opt-in form tool and publish it at the very last step when you want to get the tag. The tag looks something like this (at the top of the code provided):
You just take the URL in between the quotation marks and use that for a link in your template.