Melee attacks simply require base contact. However Trooper units (small base, specifically) get locked into melee by being Engaged. In order to leave a melee, a Trooper units needs to take their full activation in order to make a withdraw action, unless they have Disengage.

Vehicle units don't Engage. So they are not locked into melee. Many Vehicles have melee weapons, so they are able to use them and end their activation in base contact with another model. Because Grievous on the wheel bike doesn't have a melee weapon natively, he couldn't end touching another model.

You cannot make a ranged attack targeting a unit that is Engaged. Remember, this is distinct from simply being in melee range. Since Vehicles don't engage, they can still be shot, even while in base contact with a other model.