I can give you information on all raid boss mechanics (camped it first month when it came out and still do them from time to time).
All bosses scale, HP included based on party size. I can find out this information if I went in to a raid with like 1, then 2, then 3 people and then checked what the HP of vasa was (can edit this later).
Anyways, Vasa mechanics are very simple.
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From the beginning:
Vasa spawns in the middle of the room, and teleports anyone and everyone who is in it upon entering into a random location (usually best to send in fewer people so less damage is taken for whats about to happen next). Out of the random locations, one of them is the middle of the room next to it. If you get teleported to the middle of the room, your prayer is disabled and you are stunned whilst Vasa emits an AOE attack to everything in the middle.
Everyone who didn't get teleported into the middle can run into the middle with pray magic on to take 0 damage, and also mitigate the damage the person who got teled will take. Also, vasa normally attacks with range projectiles after that, that can be dodged if you keep moving as you attack (I think).
After this, everyone starts ranging Vasa as it begins to walk slowly to one of the crystals (I believe you have 30 seconds to attack it until it walks there or something).
After it reaches a crystal, it becomes invulnerable to all attacks until the crystal is dead. This process much repeats until you eventually kill Vasa (with vasa walking to a crystal, then it dies, then it attacks everyone with range slowly as it walks to the next crystal). You also have a time limit to kill each crystal, if you dont it'll do the teleport thing that it did at the beginning when you first entered where people can randomly get teleported into the middle and take damage. I believe the crystals are weak to stab attacks.
edit:
Woox's guide is very informative, too.
14:13 for him killing Vasa. (he mentions how much damage it does on the first teleport and how to dodge its attacks)
