Vanlith is a fairly straight forward encounter when your entire raid knows what to do. The basics of this encounter revolve around two main mechanics. Killing Mushrooms and Playing Hot Potato with an incurable curse. Vanlith has a 12 minute rage timer before he wipes the raid automatically.
When you start the encounter everyone in the raid will be given a new ability called Transfer Contagion so make sure your raiders know to have a empty spot on a hot bar ready to go. Everyone in your raid must have 100% reuse speed to effectively pass this part of the encounter. The recast of this ability at 100% reuse is 7 minutes and 20 seconds. Which means with a raid of 24 people you need to properly time this curse cure. However you more than likely will not have 24 people doing this mechanic. You will need at least 3 tanks during this encounter to handle the mushrooms and another curse effect so most guilds will have to skip using most of their tanks as options to pass along this curse. Which means at most say 21 people passing the curse around. You will need to cure the curse when the person that has it has less than 10 seconds remaining. If your guild or players did the Drinal fight and the curse cure on that encounter you will understand how precise you will need to be. Hopefully your raids dps can overcome the time crunch in case you hit more than 24 people passing the curse around.
Now, the way this curse works is that someone gets it and if they die with it or they don't get transferred the curse hits everyone and everyone dies. So, the easiest way to pass this curse on is to probably go right down the raid window order of how you are going to transfer it. The person who gets the curse and the person who is going to use the ability to transfer it to themselves need to be at least 15 meters away from anyone else in the raid. (This also goes for spectators..if you have anyone near your jousting spot ask them to move because at this time the curse can bug out if people are near by but NOT in your raid)
So the basics are Raid person #1 gets it and eventually moves to joust spot at least 15 meters away from any one else in the raid. Raid person #2 goes to joust spot and uses ability to transfer it. Raid person #2 now has the effect and calls out for raid person #3 to come to them to curse them at the right moment. Rinse and repeat through out the entire duration of the fight. Be sure to have your jousters warded so they don't die as they move away from the raid.
Mechanic #2. During the fight starting at 90% Vanlith will start spawning mushrooms in the area. Now depending on where the mushrooms spawn might affect your joust spot so be sure to have back up spots ready to go in case there is a mushroom near your joust spot. There is a mushroom for each archetype in the raid. (Fighters/Mages/Priest/Scouts) Each mushroom is color coordinated to your classes designation. (Red=Fighters, Mages=Blue, Priests=Green and Scouts=Yellow) The mushrooms are immune to damage until their damage shield is removed. This is removed by having your archetypes move to the mushroom and standing next to it. The closer the better. The more classes you have near the mushroom the faster the shield is removed. Anyone else within 20 meters of the mushroom will slow its vulnerability. So that means everyone needs to get away at least 20 meters. Eventually, (15 seconds or less depending on how many people are by the mushroom) you will see a message that says you can attack it. Also, when the mushrooms are up Vanlith is immune to all damage so don't waste your abilities. Once you kill the mushroom, Vanlith will be able to be damaged by one type of damage only. The first one at 90% is random so you will need to discover what it is that will hurt him (so have someone call it out) but once you know what it is the order of damage to inflict will always be Noxious, Arcane, Elemental. It will rotate just like that with one damage type available every 10%. So that means all dps in your raid, including healers and tanks will need charms that will change their damage type. So Chanters will need Noxious and Elemental changers. Warlocks will need Arcane and Elemental... and so on. Melee and Scout DPS have to choose the best they can for their class. If you do not do this you will more than likely not have enough dps to complete the encounter. This mushroom rotation will happens every 10% and then one more time at 5%.
Mechanic #3. During the fight the tank that has aggro on Vanlith will get a cureable curse on him. If this curse is cured while aggro is still focused on this tank the tank will die. So you need to set up an aggro rotation with your tanks so that when the curse lands Tank #2 picks it up right away and then the curse is cured on Tank #1. You only have about 10 seconds to get aggro swapped and then the curse cured before the tank with the curse dies anyways.
Each time someone dies Vanlith gets more increments on him and does more damage. There is a death limit on this encounter as well so basically don't die too much. If your raid executes each mechanic individually without getting too flustered then your guild will not have a problem killing this encounter.
Good Luck.