First thing to do is run the email test in admincp/maintenance/diagnostics and see if you get the email from there. Then you can know if the vbulletin email is working. You can set up SMTP in the vboptions if your provider has PHP mail shut off. But run the diagnostic email test first to see if its working. The other possibility is that your emails are ending up in the spam bucket. A LOT of FREE mail spam filter are set to block ANYTHING that looks like spam including registration verification emails.