Veteran Warleader — ruling 1

The ability that defines Veteran Warleader's power and toughness works in all zones, not just the battlefield. If Veteran Warleader is on the battlefield, it will count itself.

Veteran Warleader — ruling 2

To activate the last ability, you can tap any other untapped Ally you control, including one that hasn't been under your control since the beginning of your most recent turn. (Note that ability doesn't use the tap symbol.)

Veteran Warleader — ruling 3

You choose which ability Veteran Warleader gains as the last ability resolves, not as you activate it.

Veteran Warleader — ruling 4

Vigilance matters only as attackers are declared. Giving a creature vigilance after that point won't untap it, even if it's attacking.

Veteran Warleader : règles, fonctionnement et interactions (MTG)

Veteran Warleader 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

Veteran Warleader's power and toughness are each equal to the number of creatures you control. Tap another untapped Ally you control: This creature gains your choice of first strike, vigilance, or trample until end of turn.

How it works : Veteran Warleader

Original English text — Wizards of the Coast / Scryfall

#1August 25, 2015· Wizards of the Coast
The ability that defines Veteran Warleader's power and toughness works in all zones, not just the battlefield. If Veteran Warleader is on the battlefield, it will count itself.
No other card is named in this ruling.
#2August 25, 2015· Wizards of the Coast
To activate the last ability, you can tap any other untapped Ally you control, including one that hasn't been under your control since the beginning of your most recent turn. (Note that ability doesn't use the tap symbol.)
No other card is named in this ruling.
#3August 25, 2015· Wizards of the Coast
You choose which ability Veteran Warleader gains as the last ability resolves, not as you activate it.
No other card is named in this ruling.
#4August 25, 2015· Wizards of the Coast
Vigilance matters only as attackers are declared. Giving a creature vigilance after that point won't untap it, even if it's attacking.
No other card is named in this ruling.