The FINAL FANTASY RESONANCE list of Visions runs to 26, and all 26 are obtainable — none can be missed. Sixteen come from Sanctums, two join you in chapter 1, seven come from side quests, and one from a single optional fight.

FINAL FANTASY RESONANCE list of Visions, by role

RoleVisions
Sentinel (4)Cecil, Charlotte, Warrior of Light, Wilhelm
Commando (8)Camille, Noctis, Onion Knight, Sephiroth, Shantotto, Squall, Terra, Victoria
Breaker (10)Aileen, Bartz, Clive, Cloud, Firion, Lightning, Tidus, Tronn, Vaan, Y'shtola
Medic (2)Ayaka, Leah
Saboteur (2)Amelia, Zidane
Synergist (0)None. Nichol is the only Synergist in the whole game

Element coverage runs Fire 7, Ice 8, Lightning 8, Water 9, Wind 7, Earth 8, Light 4 and Dark 3. Those counts are Vision-element pairs rather than Visions, because one Vision can carry more than one element. Six elements are spread over seven to nine Visions each; Light and Dark are the exceptions with four and three, and only Cecil carries both.

Where all 26 come from

16 Sanctums. Each Sanctum holds three questions with no wrong answer, and each one gives a Vision:

RegionSanctumsVisions
Grandport3Clive, Cloud, Warrior of Light
Kolobos1Terra
Dirnado3Noctis, Onion Knight, Zidane
Olderion3Firion, Squall, Tidus
Zoldaad3Cecil, Shantotto, Y'shtola
Mysidia3Bartz, Lightning, Vaan

2 from the story. Leah and Tronn join with the story in chapter 1, in the Town of Mitra.

7 from side quests:

VisionQuestWhereChapter
AileenForger of WillsDilmagia5
AmeliaErrand's EndZoldaad5
CamilleThe Call from BeyondImperial Residences5
VictoriaVictoria RevivifiedMysidia5
WilhelmO General! My General!Zoldaad Castle Plaza5
AyakaThe Admirable ApprenticeTown of Sian Pharm7
CharlotteThe Shield of the WeakGrandport Harbor7

1 from a fight. Sephiroth waits at Whirlpool Hollow, the whirlpool between Grandport and Grandshelt, reachable by airship from chapter 5. He is level 45 with 250,000 HP, and you can refuse the challenge and come back later.

Every Sanctum, the whirlpool and all seven quest rewards stay open for the rest of the game, so none of the 26 can be missed.

Building a party around them

The role split above is the whole build rule, because each of your four heroes equips a Vision that brings a second role. Rain, Fina, Nichol and Dark Fina already cover Sentinel, Medic, Synergist and Saboteur on their own, so two Breaker or Commando Visions finish the six roles in a single party. The four seats and what each one should equip are on the party builds page.

How to judge a Vision

Strength is not published by the game, and the roster above says nothing about it. What does exist is a community vote board that ranks every Vision inside its own role — how that board works and how to read it is on the vision tier list page.