It doesn't matter what url format you use before converting to xenforo. Your content identifiers (thread ID, user ID, forum ID, etc.) would change anyway, after you import your forum. So even if you install vbseo and get it to mimic the XF url format, it won't help once you make the jump.
In fact, if you plan to install your xenforo forum in the same directory as your current vB forum; you need to have a different url format, for the redirects to work.