[PaulDinam]

Best thing is to make a var for player for instance:
RentVehicle[MAX_PLAYERS];

When player connects:
RentVehicle[playerid] = INVALID_VEHICLE_ID;

pawn Код:

stock IsVehicleRented(vehicle)
{
    for(new i = 0; i < MAX_PLAYERS; i++)
    {
        return true;
    }
    return false;
}

OnPlayerStateChange:
if the player is driver as a new state, use the stock above and send him the proper messages.

In /rentvehicle cmd also check if the vehicle is taken by someone else if not then just set the RentVehicle value to the vehicle id.