I have just updated the Vertex Upgrade Patch
with a minor change that I believe will stop it from defaulting to the thumbnail size.
To install the patch:
1. Download the Vertex Upgrade Patch
2. Back up your site (just in case).
3. Copy the vertex/footer.php from the theme folder to somewhere safe (you'll be putting it back after patching).
4. Delete the vertex folder from the theme (nothing else, that is the only
thing to delete).
5. Extract the zip file containing the patch, and upload it to the theme folder through FTP, choosing "overwrite" if/when prompted.
6. Copy the footer.php file (from step 3) back into the vertex folder of the theme.
Or, to put it a different way:
1. Back up vertex/footer.php.
2. Delete the vertex folder, then upload the extracted patch to the theme folder.
3. Put the footer.php file back into the vertex folder of the theme.