As you mentioned there is an additional requirement of MySQLi for XenForo to be installed. I'm really feeling very bad that my host is not installing it for me, they said they have 500 clients and no one demands MySQLi support, this is only you. If you need it for your own hosting upgrade to VPS or dedicated server.
I already paid my host for two years of hosting and I can't leave them also can't afford VPS or dedicated server. So why did you require extra server resources? my vbulletin installation is working just fine without mySQLi. why there is not an alternative solution?