I've had a few party members ask me about how this works, so I figured I would post it here to make sure that everyone understands and can plan accordingly when building their setups with these medals.
Illustrated Vivi's special ability is as follows:
Raises strength by 3 tiers for 3 attacks. Raises strength of all attributes by 3 tiers for 3 attacks. Restores 5 gauges.
The ability includes its own attack, so using this medal's special ability would buff itself and two attacks after it. Note that this says ATTACKS, not medals. This is an important distinction now that the Extra Attack trait has changed the way that medals like this will function. Let's put out a few examples:
Ivivi buffs itself in slot 3, the attack of the medal in slot 4 with no Extra Attack, and the attack of the medal in slot 5.
Ivivi buffs itself in slot 3, the attack of the medal in slot 4, and the extra attack in slot 4.
Once Extra Attack is added to the medal, this begins to get a little bit more confusing. The first cast will buff itself, the second cast, and the next attack. The second cast will buff itself, and the next two attacks. This means that only one attack (that is not Ivivi itself) will benefit from the full +6 GAU and +6 PSM. Here's an example:
Ivivi in slot 3 with extra attack. The first cast will give +3 buff to itself, the second cast of Ivivi, and one attack after that (whatever attacks in slot 4). The second cast will buff itself (for a total of +6), then the next attack for a total of +6, then the following attack for +3. So your first attack in slot 4 will have a +6 attack, and (if your medal has Extra Attack), the second attack will have +3. Otherwise, slot 4 gets +6 and slot 5 gets +3.
Here's my recommendation: if you roll Extra Attack on Ivivi, put it in slot 3 of a keyblade like MoG or Olympia. Since slot 4 has the highest multiplier on these keyblades, giving that attack with your strongest medal in the 4th slot the highest buff. For instance, if you have HD Ursula, putting her after Ivivi on MoG will hit the hardest from slot 4.
Confused? Maybe. Ask questions if you have them!