Make a copy of your vBulletin site on a different server (or in a different location of your current server) or using WAMP and make a test conversion. If everything goes well then you can either make the test conversion your actual production website or you can go ahead and make the conversion of the live website once you know everything will work alright (and with the peace of mind that you have the other converted site in case something goes wrong.)
Of course, make backups of everything before doing anything.