plugins are strictly a Roland and/or Roland collaborated phenomenon. They are only available through the use of the various FX cards, which are installed in the body of the VS recorder. The cards can still be found at places such as ebay, or reverb, but are not being produced anymore and as far as I know are not available for purchase from Roland.
You could always use a pc/mac daw, with whatever plugins you have, as a processing tool - sending tracks out of the Roland into the daw and then looped back into the recorder. Or you could export and import wav files from and to the recorder, to be processed in a pc/mac computer. That's about the size of it though.