What may be easier is to copy the contents of the custom fields in vBulletin to the default ones in XenForo via a My SQL query.
The thing is, those default fields are called in lots of templates so removing them may cause template errors. I'm not sure, I haven't tested it.
But, the thing is, those fields - they're all default ones in vBulletin, aren't they? Why did the end up being "custom"?
If they are the default ones, the data will automatically copy over to the XenForo ones.