This truly is a moot point. Really semantics. There really is no unlimited plan anymore with VZW. If you are on an unlimited plan, all this means is you won't get charged any extra if you go over, but they will throttle you if you go over 2gb. If you continuously use over 2gb you may be permanently throttled or throttled for an extended period of time. If you do not have an unlimited plan and you go over, you get charged hand over foot per mb.
So in reality it's just a semantics question anymore. The cap is 2gb. Unlimited or Not. The difference between is what happens if you go over that 2gb.
Personally, I've always used Wifi mostly to save battery life and speed. It's funny how VZW is embracing Wifi use, when they used to frown upon it. They wanted to charge for everything so they used to block blackberry from using wifi. Now they encourage you to use wifi so they can sell more smartphones without bogging down their network.
And yes, when on Wifi, you stop using VZW data. Only data you pull through their network count against your usage.