There's an IPB converter available, so you can go vBulletin 5 > IPB > Xenforo. If you don't own a copy of IPB, there are many members here who offer import services. You could try asking them if they do vB5 conversions.
If you can't get your head around this, then try hailing @Jake Bunce - he's our resident Import guru. As Mike stated, make sure you're using the correct IPB importer version along with the latest version of XenForo.