Only fields 1 - 4 in the source are imported into core fields. Unfortunately, there is no way to identify what those fields are, so if you want to change the structure to import those into custom fields, you would need to modify the code to fit your exact needs. (Fields 1-4 are default fields in the source DB.)
I'm not talking about importing vBulletin's custom user fields as custom user fields on XenForo.
See, you're already importing the contents of the custom user fields inside the 'About You' section of XenForo. My proposal is to not only import the contents of those user fields but also their title, so in the 'About You' section the end result will be: