Bad news first: There is currently no vamp plugin for chord extraction that I am aware of. That's a bit of a shame, especially since I am actually doing research on chord extraction from audio. But my stuff generally too complicated for me to try and rewrite it all in C++ for the Vamp plugin.
Secondly: Please be aware that the key extraction plugin is no chord extraction plugin. Keys describe the tonality of a chunk of music, which usually contains many chords, so there is quite some difference between the two concepts. Consequently, you cannot expect the key extractor to give you good chord estimates (or indeed good beat estimates
There may be hope though, since I know that we (Centre for Digital Music at Queen Mary) are going to release Vampy soon (should be a matter of hours or a few days), and I could relatively easily write a Vampy plugin for you for a very basic chord estimation. So, watch the news about Vampy and make it work on your system. Then tell me, and I'm sure we can come up with something.