Like I said above, you CANNOT map a URL with "wordpress.com" in it to any external server. Do not purchase anything, because it won't make any difference. You can't do it.
If you'd purchased the domain upgrade in the past and gotten vop-say.co.cc mapped to your WP.com blog, then you could move vop-say.co.cc to point to a different server, no problem and all the links that had been made to that URL when it was mapped to WP.com would still work and when people put that URL in, it would go to your new server. But vopchange.wordpress.com would still be here, will still always point here.
If people buy the domain upgrade early in a blog's life, it means that most if not all of the links to it will point to the new domain and thus be transferable if you ever move off WP.com. Since you didn't do that, you don't have that option. You have only one URL, not two, pointing to your WP.com blog and they do not let people redirect off WP.com.
My best suggestion, and it works pretty well, is to delete the CONTENTS of the WP.com blog (do not delete the blog itself, DO NOT DO IT!) once your new one is up and running, but set a static page to be your front page and on it put a note about your new URL and a big, prominent link to it.