always having my BitTorrent client on with a randomized open port allowing folks to connect to my machine.
What is it that your worried about? Crackers owning your machine, or people finding out what you're downloading?
If the former, your setup can be made fairly secure (configure your firewall to only allow incoming connections to your BT port, run your BT client under a separate UID in a chroot jail, etc.).
If the latter, this cannot be done with BitTorrent. Always assume that your ISP, your local law enforcement agency, and your girlfriend know exactly what you are downloading. (Girlfriends usually do.)
I've been looking at setting up a virtual private network for (more) secure data transfers
In which case you're adding yet another entity who knows what you're downloading -- the VPN company.
there's no intent on doing anything illegal on my part
That's none of our business -- nobody here has any right to tell you what you should and what you shouldn't download. It's purely a local matter between you and your law enforcement agency.
--Juliusz