Format rules
Rules
Vanillamon is defined by one test: a legal Pokémon has attacks without printed rules text.
The core rule
Only Pokémon cards whose attacks have no text are allowed. An attack name, attack cost, damage number, and GX reminder text in brackets do not count as attack text for this format.
Plain test: if there are words under the attack name explaining what the attack does, the Pokémon is not legal in Vanillamon.
Card pool and deck construction
All sets legal in the SUM-on format are legal in Vanillamon before the banlist is applied. Deck construction uses the same limits as Expanded.
The Pokémon restriction is severe, but all other cards are legal unless they appear on the Vanillamon banlist.
Abilities are allowed
Abilities are not attacks. A Pokémon with strong abilities can be legal as long as its printed attacks pass the no-text test.
This is where much of the format depth comes from: ability engines can define a deck even when the attacks are intentionally simple.
Rule Box Pokémon can be legal
Rule Box Pokémon are allowed when their attacks pass the same test. GX attacks also follow the test, but bracketed GX reminder text is ignored.
What counts as text?
Any printed words describing the attack count as text. Coin-flip instructions, bench damage, status conditions, discard clauses, search clauses, and similar instructions all make the Pokémon illegal, even when the game rules might describe those words as part of damage calculation.
Legal examples from the manifesto
Ability engines

Flaaffy (EVS 55)Evolving Skies

Metang (TEF 114)Temporal Forces

Inteleon (CRE 43)Chilling Reign

Drizzile (SSH 56)Sword & Shield

Zoroark (EVS 103)Evolving Skies

Cherrim (BST 8)Battle Styles
Rule Box and special cases

Gyarados GX (SHF 16)Hidden Fates

Charizard GX (SHF 9)Hidden Fates

Tapu Koko Prism Star (TEU 51)Team Up