I'm only specifically referring to the difference between the tariffs.
As for tethering, you are not 'supposed' to do it now no. Even if you have unlimited data. Also once you hit 3.5gb usage in a month your speed is restricted.
http://forums.digitalspy.co.uk/showthread.php?t=1790721
edit - If you want more detail have a search for the fair use policy on the VM mobile site.
http://www.virginmobile.com/vm/gener...e.footer.sm068