Technically speaking, we dont have 2g. We have 1x, and then EVDO. 1x is somewhat of a 2g technology, but its not a different radio. EvDo, as its simply a software update to the modems, is available ANYWHERE that Verizon has 1x. The only places you would be able to use 1x is if you were in a very iffy signal location, where the speed at which your phone can communicate with the tower is so low that EvDo cant kick in, or while roaming on sprints towers. There is no actual way to enable 2g only like you can on GSM phones, and since its the same radio, it uses the same power, so theres no benefit in turning 2g only on anyway with a CDMA phone. 1x will actually cause your battery to drain FASTER, if anything, due to it having to stay active longer to download your data.
When you turn off 3g on the phone, you are actually turning off mobile data, not EvDo. As in, your phone wont attempt to connect to the internet via Verizon over the cell network. It uses 1x in that case for calls and texts only.