CDMA and GSM are two different types of signals and a phone is either one or the other, not both. However, some apps may be able to be used on both types of phones.
If you don't want to use 4G, don't turn it on. Then it only uses 3G. And yes, 4G uses more battery than 3G does.
The apps have no idea whether you're CDMA or GSM, they just want to use a data network.
Regarding the 3G/4G thing, there is a ton of confusion. In the AT&T world 3G and 4G are really the same thing (the same wireless network). So on the Vivid there is no separation between 3G and 4G.
Some phones have the ability to turn 3G off and use only 2G so save power. I'm not sure about the Vivid. I'll look into it.