Vehicles and Monstrous Creatures must be hidden in appropriate terrain features. It is not possible to disguise a Leman Russ battle tank on an open plain, but it could be hidden at the edge of a wood, behind a building or behind a hill. Vehicles and MCs 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 And Monstrous Creatures
Source: 3rd-A-Palooza (3.5) Rulebook v1.4, p. 60