So as suspected, Vegas 13 has a very poor implementation of the OFX standard with many bugs, even with the test plugins that come with the OFX SDK. I managed to work around most of them now, I think.
Here is a new release: http://reduxfx.com/gmic/20190218_gmic_ae_ofx.zip
@MKdmn: I think the issue you are seeing is not related to that, as the plugin should have been showing up in the list regardless of the render issues already. Probably there is an issue with the libcgmicstatic.dll. Try the following steps:
Search your hard drive for any existing libcgmicstatic.dll occurences and delete/rename those files.
Then run a command prompt (“cmd”) and type “path”. It will list all the directories in the path variable. Put the libcgmicstatic.dll from the archive linked above into any of the directories listed (just the DLL directly, not in a subfolder)