Using Safari gives you the mobile version?

Anyway, vB4 had a mobile version available and you may set it up so that users using a mobile device will automatically view the site using the mobile style. You may test/see it on vbulletin.com/forums

vB5 uses a responsive style which will be good for browsers and mobile devices. You may test/see it on http://www.vbulletin.com/vb5demo/

Given your situation you will probably need help. If your forum is large you want to make sure all inbound referral traffic is redirected properly.

Example:

www.yoursite.com/dragonflythread1234

Needs to be redirected to;

https://vborg.vbsupport.ru/showthread.php?t=1234

Otherwise you will lose all referral traffic. This shouldn't even an issue if you don't mind losing the traffic.

VB in itself is pretty easy to set up and importing other boards is relatively straightforward too.