More than levelness, "flatness" is a much more important factor. In other words, if your entire room slopes in one direction, but is otherwise flat the entire way, your installation can be successful.
The recommended flatness is 3/16” within 10 ft.
That said, we have had many customers tell us they have installed on considerably less flat surfaces. There are three primary concerns with the subfloor not being flat.
The first is that the click mechanism may come loose. If the subfloor changes angle too dramatically in the middle of a plank, the pressure over time may lead to the click mechanism breaking or coming loose.
The second concern is telegraphing. Precisely because vinyl floors are malleable, you can have imperfections in the subfloor telegraph to the surface and be visible.
The final concern is hollowness. When a plank bridges a valley, the hollow spots can result in the floor sounding soft or "loose" wherever there are air pockets between the floor and the subfloor.
Checking for flatness prior to your installation is the most important step you can take for a successful install.