Viper Gang ambush does not have a map marker, so there is no way of fast-traveling to its location. However, when following the east road out of Nipton, you'll get there. Other map markers nearby include Caesar's Legion Safehouse and Wolfhorn Ranch.
Once you get in the vicinity of the ambush, you'll be spotted by a few members of the Viper Gang. They will start attacking immediately. Note that some of the Viper Gang will be up on the cliff, and can be quite hard to reach.
Furthermore, there are a few wrecked cars and some hidden frag mines, but nothing more.