Vivien, Arkbow Ranger — ruling 1

You choose how the counters will be distributed as you activate Vivien’s first ability. Each target creature must be assigned at least one counter. This means that you can’t put two counters on one creature but give two creatures trample.

Vivien, Arkbow Ranger — ruling 2

You can activate Vivien’s first ability without choosing any target creatures. The counters won’t be put on anything. This is a change from previous rules regarding distributing counters.

Vivien, Arkbow Ranger — ruling 3

If one of the two target creatures becomes an illegal target in response to Vivien’s first ability, the +1/+1 counter that would have been put on that creature is lost. It can’t be put on the legal target.

Vivien, Arkbow Ranger — ruling 4

If either target is an illegal target as Vivien’s second ability tries to resolve, the creature you control won’t deal damage.

Vivien, Arkbow Ranger — ruling 5

In a casual game, a card you choose from outside the game comes from your personal collection. In a tournament event, a card you choose from outside the game must come from your sideboard. You may look at your sideboard at any time.

Vivien, Arkbow Ranger : règles, fonctionnement et interactions (MTG)

Vivien, Arkbow Ranger 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

+1: Distribute two +1/+1 counters among up to two target creatures. They gain trample until end of turn. −3: Target creature you control deals damage equal to its power to target creature or planeswalker. −5: You may reveal a creature card you own from outside the game and put it into your hand.

How it works : Vivien, Arkbow Ranger

Original English text — Wizards of the Coast / Scryfall

#1July 12, 2019· Wizards of the Coast
You choose how the counters will be distributed as you activate Vivien’s first ability. Each target creature must be assigned at least one counter. This means that you can’t put two counters on one creature but give two creatures trample.
No other card is named in this ruling.
#2July 12, 2019· Wizards of the Coast
You can activate Vivien’s first ability without choosing any target creatures. The counters won’t be put on anything. This is a change from previous rules regarding distributing counters.
No other card is named in this ruling.
#3July 12, 2019· Wizards of the Coast
If one of the two target creatures becomes an illegal target in response to Vivien’s first ability, the +1/+1 counter that would have been put on that creature is lost. It can’t be put on the legal target.
No other card is named in this ruling.
#4July 12, 2019· Wizards of the Coast
If either target is an illegal target as Vivien’s second ability tries to resolve, the creature you control won’t deal damage.
No other card is named in this ruling.
#5July 12, 2019· Wizards of the Coast
In a casual game, a card you choose from outside the game comes from your personal collection. In a tournament event, a card you choose from outside the game must come from your sideboard. You may look at your sideboard at any time.
No other card is named in this ruling.