Would it be possible for you to rephrase your question – I’m not really sure what you’re asking.
Which forum do you want to buy?
You can set up this forum in a different directory to your current blog and you’ll have no problems at all. If, however, you have a blog on your current WordPress setup, you won’t be able to transfer the data across.
Damn, i have wordpress..!! Not is possible?
You can use your blog on WP, and the forum on vB. But, if you already have a WP forum you can’t transfer the data.
So then you won’t be able to transfer the data to the new one I don’t think.