As Kerry-Anne said, the size of your forum is key here. If you have a big board on cheap hosting, it doesn't matter what forum software you run, it will be slow.

Other factors are how, exactly, your shared host is set up. Which PHP version, what webserver, etc, etc. There are a ton of factors that can lead to slow performance, and imo vBulletin being the cause is way, way down on the bottom of that list.

Hogwash. PHP is as secure or insecure as you make it. Nothing is completely hack-proof, but even with the most basic security policies in place, you're good to go against 90% of the routine script-kiddies out there. You just got a lazy tech on the phone that didn't want to help you and threw you a cop-out to get you off of the phone.