Yeah I am having the same problem too. In certain interior locations I'm also disappearing. The locations of where they are happening are in Camp McCarran Building by Curtis' office and the Science Room, Kings School in Freeside in the Hallways that go upstairs, Parts of the Hidden Valley Brotherhood Bunker around Eldar McNamara's room in a hallway with rooms and Parts of Gomorrah in the Suites. These are the only places I've really only experienced turning invisible. It would really help for a solution because the turning invisible part is really ruining immersion and experience.
Since I also use ENB, they claim ENB is responsible and that the only response I get is turn off TransparencyFix as well as AmbientOcculsion but to no anvil it doesn't work. Really I am desperate for a solution and I would honestly be thankful for any solution.
EDIT: I found a solution uninstall and reinstall the Game! IT WORKED!