If you're buying a used modern Vespa scooter, it is imperative that you get both keys: the blue key and the red (sometimes called "brown") key. The absence of the red key is a major problem, and you should consider walking away from any sale where the red key isn't presented up-front.
The red key is the master key for any Vespa or Piaggio scooter outfitted with an immobilizer. The red key is used to program the scooter to recognize and accept the blue key(s), each of which has their own RFID code. Without the red key, you generally can't add additional keys, and if you ever lose your blue key, you'll be completely unable to start the bike ever again without replacing some fairly expensive parts.
The ECS (Engine Control System) can be replaced, along with the ignition lockset. The cost of this replacement will be somewhere around $450 for parts, not including labor. The new ECS will come with a new red key with a new physical cut (thus requiring a new ignition lockset). Once the new ECS has been installed, the scooter can be programmed to recognize newly-cut blue keys.
Cloned Blue Key
If the blue key is still available, it can be cloned so that the new key carries the same RFID code as the existing blue key. This means that the scooter will be unable to distinguish between the keys, and so both will work. This is considerably less expensive than replacing the ECS, although there's an outside chance that the ECS might "forget" the ID of the blue key at some point, rendering both original and cloned blue keys useless. The chances of this are slight, but still cause for concern.