How do I counter the Advent Rebel’s Wail of the sacrified as a Vasari Rebel ?
Well if you're playing against the AI, it doesn't use Wail of the Sacrificed. And if you're online, I don't think anyone will let you use Vasari Rebel. 
How do I minimize the damage done by this ability ? Split my fleet in 2 groups ?
The fact that wail destroys the planet, and the fact that population takes a long time to regrow, means timing is everything with wail. You don't get a second shot to use it in the near future. This means the AR player is going to be on substancial preasure to use it for a killing blow. If you keep your fleets in groups so that he never gets to see a killing blow, he'll either have to use wail on something you can replace or not use it at all.
What's Wail's range: only the current planet or the current planet + all the other planets ?
Every planet adjacent to the planet he has wail on. This DOES NOT include the planet itself, so once you are at the planet with Wail, you are safe from that wail (things get nasty when multiple planets with wail are next to each other though).
Once he launches Wail, how much time do I have ? What warning do I have ? Is it instant ?
Not much. The only clue you'd have is on the planet wail is at in the form of a particle effect, but you'd need to be scouting it already, and its not really enough time to do anything about it.
So the Advent rebels just have to keep a colonizer near that scarified planet population and can wipe out my whole fleet many times?
Again, the planet needs to replace the population first. That will take a fair amount of time, so as long as you don't let it take out your whole fleet you should be able to do something about it.
“Deals 20 damage per point of population, sacrifices entire population of planet.” How is that damage distributed on my ships ? (does it do 20x population on all my ships ???)
Yes. A planet with 100 population will do 2000 damage to all enemy ships in planets adjacent to the planet that used wail (but not including that planet itself). This is raw damage though, armor and mitigation will reduce this somewhat.
So maybe I could use my endgame canon to neutralize the 2 planets next to the one I want to attack? But then maybe I would need 3? One for the planet I want to attack, one for each neighboor ?
The Novaltih Cannon and Kostura both are great against Wail. The Kostura disables all structures at the planet it hits, so your opponent will not be able to use for 3 minutes after a Kostura strike. Also, directly phase jumping to a planet with wail via Kostura allows you to bypass it completely, as the planet cannot wail itself. You just have to be careful that there is not another planet that also has wail on it next to it.
Finally, if the planet isn't that well defended, just don't send any frigates to attack. Caps, Titans and Starbases (as the VR you have a huge advantage with this) cannot be killed with a single wail attack. Once you've destroyed the culture centers with them, you're free to send in the rest of your fleet, hopefully before reinforcements arrive.
