What about: 
function createVehicleAndWarp (source, command, vehNAME)
local x, y, z = getElementPosition (source)
local vehID = getVehicleModelFromName(vehNAME)
local vehicle = createVehicle (vehID, x, y, z)
warpPedIntoVehicle(source, vehicle)
end
addCommandHandler("vehicle", createVehicleAndWarp)
 
/vehicle nrg-500  
^^