I'm not sure but I think PA9 was the first version to include DX support and no version of ProAudio ever supported VSTs. At one time there was a Cakewalk wrapper that allowed PA9 to run VSTs but never VSTi.
I don't think the wrapper is even available from Cakewalk any more. Google might help you find a 3rd party wrapper for a VST but I'd be surprised if you find something to work with PA6. It's really, really old.
--Larry