Vehicles must be hidden in appropriate terrain features. It is not possible to disguise a Leman Russ battle tank on a open plain, but it could be hidden at the edge of a wood, behind a building or behind a hill. Vehicles must be hidden behind or in a terrain feature which could conceivably hide the model. Because people's terrain varies so much it is impossible to give an exhaustive list, so apply common sense. Obviously, it's much easier to hide a buggy than a tank.
Hidden Vehicles
Source: Core Rulebook 3E, p. 134
Under Hidden Set-up, vehicles must be concealed behind or in a terrain feature that could plausibly hide them.