In short, they didn't really Upgrade much but the Aesthetics.
Joyetech just essentially refreshed their Evic line-up. They gave a bunch of new color options to both the VT and the VTC Mini (not all the VTs have the race stripe design anymore). Check out the links to see all the color options available, plus specs.
http://www.joyetech.com/product/evic-vtwo/
http://www.joyetech.com/product/evic-vtwo-mini/
They also added the RTC (Real Time Clock) feature, allowing use of a clock face for the screen (analog clock whole/half screen, and digital time/date display).
The one ACTUAL Hardware Upgrade was they gave it
Dual Circuit Protection, for safety redundancy (to help prevent possible overcharging issues).
The mod was pretty much perfect before, there wasn't a whole lot to change
Edit: It appears they may have also upgraded the 510, unconfirmed though