Vivien, Monsters' Advocate — ruling 1

Vivien lets you look at the top card of your library whenever you want (with one restriction; see below), even if you don't have priority. This action doesn't use the stack. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand.

Vivien, Monsters' Advocate — ruling 2

If the top card of your library changes while you're casting a spell, playing a land, or activating an ability, you can't look at the new top card until you finish doing so. This means that if you cast the top card of your library, you can't look at the next one until you're done paying for that spell.

Vivien, Monsters' Advocate — ruling 3

Normally, Vivien allows you to cast the top card of your library if it's a creature card, it's your main phase, and the stack is empty. If that creature card has flash, you'll be able to cast it any time you could cast an instant, even on an opponent's turn.

Vivien, Monsters' Advocate — ruling 4

You'll still pay all costs for that spell, including additional costs. You may also pay alternative costs such as mutate costs.

Vivien, Monsters' Advocate — ruling 5

The top card of your library isn't in your hand, so you can't cycle it, discard it, or activate any of its activated abilities.

Vivien, Monsters' Advocate — ruling 6

After activating Vivien's last loyalty ability, the delayed triggered ability it creates will put a creature card onto the battlefield before the creature spell that caused the ability to trigger resolves.

Vivien, Monsters' Advocate — ruling 7

For spells with {X} in their mana costs, use the value chosen for X to determine the spell's mana value.

Vivien, Monsters' Advocate — ruling 8

If a card in a player's library has {X} in its mana cost, X is considered to be 0.

Vivien, Monsters' Advocate : règles, fonctionnement et interactions (MTG)

Vivien, Monsters' Advocate 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

You may look at the top card of your library any time. You may cast creature spells from the top of your library. +1: Create a 3/3 green Beast creature token. Put your choice of a reach counter, a vigilance counter, or a trample counter on it. −2: When you next cast a creature spell this turn, search your library for a creature card with lesser mana value, put it onto the battlefield, then shuffle.

How it works : Vivien, Monsters' Advocate

Original English text — Wizards of the Coast / Scryfall

#1April 17, 2020· Wizards of the Coast
Vivien lets you look at the top card of your library whenever you want (with one restriction; see below), even if you don't have priority. This action doesn't use the stack. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand.
No other card is named in this ruling.
#2April 17, 2020· Wizards of the Coast
If the top card of your library changes while you're casting a spell, playing a land, or activating an ability, you can't look at the new top card until you finish doing so. This means that if you cast the top card of your library, you can't look at the next one until you're done paying for that spell.
No other card is named in this ruling.
#3April 17, 2020· Wizards of the Coast
Normally, Vivien allows you to cast the top card of your library if it's a creature card, it's your main phase, and the stack is empty. If that creature card has flash, you'll be able to cast it any time you could cast an instant, even on an opponent's turn.
No other card is named in this ruling.
#4April 17, 2020· Wizards of the Coast
You'll still pay all costs for that spell, including additional costs. You may also pay alternative costs such as mutate costs.
No other card is named in this ruling.
#5April 17, 2020· Wizards of the Coast
The top card of your library isn't in your hand, so you can't cycle it, discard it, or activate any of its activated abilities.
No other card is named in this ruling.
#6April 17, 2020· Wizards of the Coast
After activating Vivien's last loyalty ability, the delayed triggered ability it creates will put a creature card onto the battlefield before the creature spell that caused the ability to trigger resolves.
No other card is named in this ruling.
#7April 17, 2020· Wizards of the Coast
For spells with in their mana costs, use the value chosen for X to determine the spell's mana value.
No other card is named in this ruling.
#8April 17, 2020· Wizards of the Coast
If a card in a player's library has in its mana cost, X is considered to be 0.
No other card is named in this ruling.
#9April 17, 2020· Wizards of the Coast
If you cast a mutating creature spell, you must choose a target for it before the triggered ability from Vivien's last ability puts a creature onto the battlefield. The creature you find can't be the target of the mutating creature spell.
No other card is named in this ruling.