While Sn0wbreeze 2.7.1 can already jailbreak iOS 4.2.8 on Verizon iPhone 4, the problem is that it is for Windows users only. If you happen to be a Mac user, and want to untether jailbreak your Verizon iPhone 4 on iOS 4.2.8, you can do so now with this PwnageTool bundle.
The instructions for jailbreaking iOS 4.2.8 using custom PwnageTool bundle are exactly similar to the ones already covered for iOS 4.3.3 (GSM iPhone 4). Simply download iOS 4.2.8 from here, and iOS 4.2.8 PwnageTool bundle from here, and then follow the instructions posted here, replacing iOS 4.3.3 related stuff with iOS 4.2.8 (duh!).