XenForo is still beta, so it is not recommended to run it as a live site.
Is your vB 3.7/3.8? As of this time that is the only version of vB the importer will work on.
The approach I would take, if I read your question correctly:
From CPanel create a new DB for XenForo
Back-up my vB site, including the DB
Rename my vB's folder
Create a new folder with the old name of the directory I just renamed
Upload the XenForo files into the new directory. Add in the 301 redirect files.
Access your site from your browser (to begin the installation).
From XenForo AdminCP: Tools -> Import External Data, to begin the importing of your 3.7/3.8 DB.
This way if anything went wrong with the Importing, just rename the XF directory to something different, and rename your vB directory back to the original name.