Prime Speaker Vannifar — ruling 1

If a creature or a creature card in your library has {X} in its mana cost, X is considered to be 0.

Prime Speaker Vannifar — ruling 2

Tokens that aren't a copy of something else don't have a mana cost. Anything without a mana cost normally has a mana value of 0.

Prime Speaker Vannifar : règles, fonctionnement et interactions (MTG)

Prime Speaker Vannifar is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

, Sacrifice another creature: Search your library for a creature card with mana value equal to 1 plus the sacrificed creature's mana value, put that card onto the battlefield, then shuffle. Activate only as a sorcery.

How it works : Prime Speaker Vannifar

Original English text — Wizards of the Coast / Scryfall

#1January 25, 2019· Wizards of the Coast
If a creature or a creature card in your library has in its mana cost, X is considered to be 0.
No other card is named in this ruling.
#2January 25, 2019· Wizards of the Coast
Tokens that aren't a copy of something else don't have a mana cost. Anything without a mana cost normally has a mana value of 0.
No other card is named in this ruling.