you can buy them off someone who bought it. They'd have to sell you the license for it as well as not use it anymore themselves. You cant buy it from vb anymore. However they'd be so full of security holes that wouldnt have been addressed in years so its worthless.
I've seen some very large forums out there still using them so I guess security holes could be patched somehow... but you're right, probably wouldn't be worth the hassle... just thought it might be cool to have a 'retro' day at my forum.
I'd imagine they must be pretty handy coders by this point, vb php files aren't very hard to follow and earlier versions must be even easier- the less features the less security risks in general.
Don't forget the software being used to power that site must be pretty ancient in itself also, and probably long ago EOL'd and vulnerable to a hole or two.
I'm not sure how many versions you can go back and download from the members area, but you should be able to find some there.