I would try something like this:

local vehicleModels = {
    "blista",
    "faggio",
    "comet"
}

function getRandomVehicleModel()
    local randomIndex = math.random(1, #vehicleModels)
    return vehicleModels[randomIndex]
end

and then

local vehicleModel = getRandomVehicleModel()
local player = GetPlayerPed(-1)
local x, y, z = table.unpack(GetEntityCoords(player))
local heading = GetEntityHeading(player)

local vehicle = CreateVehicle(GetHashKey(vehicleModel), x, y, z, heading, true, false)

I did not try it out, but it should work.